～かもしれない vs ～にちがいない: Possibility and Certainty in Japanese

～かもしれない and ～にちがいない sit at the two endpoints of the Japanese certainty scale. かもしれない floats a possibility the speaker has not ruled out, while にちがいない declares a near-certainty the speaker has reasoned themselves into.¹² Both are JLPT N4 grammar to reach for after でしょう/だろう and はず. Both encode the speaker's own mental judgment rather than something heard or seen.³

Overview

What the two markers do

かもしれない marks a possibility the speaker has not excluded. The standard learner gloss is "may; might; possibly; perhaps; maybe."¹

にちがいない marks a near-certainty reached by inference. The standard gloss is "there is no doubt that; no doubt; must (be); I'm sure."²

Both belong to the epistemic modal class in Aoki's foundational typology of Japanese modality. In plain terms, they encode the speaker's own mental judgment, not a perceptual or reportive source.³ That property separates both markers from そうだ (hearsay), らしい, ようだ, and みたい, which are evidential markers that signal where the information came from.

Epistemic, not evidential

かもしれない and にちがいない say something about the speaker's mind: "I judge this is possible / certain." そうだ, らしい, ようだ, and みたい say something about the evidence: "I see / hear / read that this is so." Mixing the two classes is the most common mistake in this corner of the grammar.³

Niwasaburoo frames にちがいない's meaning as close to きっと～である ("surely it is the case that…"), which captures the conviction without overstating it as a flat assertion.⁴

The certainty scale at a glance

A widely circulated four-step teaching scale places the two markers at the endpoints, with two steps between them. Kotoba-Note assigns the heuristic percentages 30–50% for かもしれない, 60–80% for だろう, and 90%+ for にちがいない. It explicitly cautions that these are one author's mental scale, not a measurement.⁵ LTL's English-language treatment uses the same ordering and characterises にちがいない as "strong certainty… through subjective thinking."⁶

Niwasaburoo's more academic treatment refines the picture. はず is a conclusion drawn from logical reasoning, and the speaker can hold onto it even against contradictory evidence. にちがいない is a subjective conviction usually built from circumstantial cues the speaker has just registered.⁴ Both sit above だろう on the conviction axis, but for different reasons.

The percentages are a teaching crutch

The 30 / 70 / 90 numbers are useful for a first orientation only. Choice in real use is driven by stance (am I hedging, or am I committing?) and register, not by arithmetic probability.⁵⁴

JLPT level and register range

かもしれない is N4 on both major learner-facing references.⁷⁸ Its register spread is unusually wide for a single grammar point. It runs from the texting-register reduction かも⁹ through the polite-spoken かもしれません⁸ and into the literary かもしれぬ.¹⁰

にちがいない is editorially N3 in current learner references¹¹¹², but this article treats it at N4 on pedagogical grounds. A learner who already knows でしょう/だろう and はず at N4 needs にちがいない to complete the scale, and the form's morphology requires no machinery beyond N4. Its register skews formal and written. In casual spoken Japanese, it is meaningfully less common than かもしれない.¹¹

Form and attachment

～かもしれない: attachment table

The structural rule is plain form + かもしれない. DBJG records it as "Sinf + kamoshirenai," meaning the predicate appears in its informal (plain) form, present or past, affirmative or negative.¹ The one trap comes after a noun or な-adjective: the copula だ drops.

Predicate type	Attachment	Example
Verb (plain form)	verb + かもしれない	行くかもしれない7
い-adjective	い-adj + かもしれない	高いかもしれない7
な-adjective	stem + かもしれない (no だ)	元気かもしれない7
Noun	noun + かもしれない (no だ)	学生かもしれない79

Bunpro states the rule explicitly: "The だ copula drops before かもしれない."⁷ Maggie Sensei demonstrates with 犬かもしれません, not ✗犬だかもしれません.⁹

Negation and past stay on the predicate, not on the modal itself. In standard use, the modal does not conjugate for tense or negation.¹⁷

午後ごごから雨あめが降ふるかもしれない。⁸
"It might start raining in the afternoon."

病気びょうきかもしれない。⁸
"I might be sick."

走はしれば終電しゅうでんに間まに合あうかもしれません。⁸
"If I run, I might make the last train in time."

そういうことを言いわないほうがいいかもしれない。⁷
"It might be better not to say things like that."

～にちがいない: attachment table

The structural rule mirrors かもしれない: plain form + にちがいない, with the same だ-drop after a noun or な-adjective.²¹¹ Bunpro makes the same point explicitly: にちがいない "attaches to plain forms of verbs, い-adjectives, な-adjectives, and nouns (without だ)."¹¹

Predicate type	Attachment	Example
Verb (plain form)	verb + にちがいない	行くにちがいない11
い-adjective	い-adj + にちがいない	高いにちがいない11
な-adjective	stem + にちがいない (no だ)	元気にちがいない11
Noun	noun + にちがいない (no だ)	学生にちがいない11

Negation and past again stay on the predicate: 知らなかったにちがいない ("must not have known"), 行かなかったにちがいない ("must not have gone").²

あの顔かおは日本人にほんじんに違ちがいない。¹¹
"With a face like that, that person must be Japanese."

彼女かのじょは病気びょうきに違ちがいない。¹¹
"She must be sick."

何なにかあったに違ちがいない。¹¹
"Something must have happened."

大雪おおゆきが降ふって大変たいへんだったに違ちがいない。¹¹
"After that heavy snowfall, it must have been rough."

にちがいない sits at N3 in current references

にちがいない is listed at N3 on Bunpro¹¹ and on equivalent learner references¹². This article treats it at N4 alongside かもしれない so the certainty scale can be taught as one unit. A learner preparing for the JLPT should expect to encounter にちがいない in N3 reading passages.

Register ladder for ～かもしれない

かもしれない has the widest register spread of any modal suffix at this level. The ladder below combines Bunpro's and Maggie Sensei's treatments.

Rung	Form	Register
Very casual	かも	sentence-final reduction; texting and friend-speech; "the most common" colloquial form79
Casual softening	かもね	"yeah, maybe" agreement-style replies; very casual9
Casual contraction	かもしれん	rough or boyish flavor; some regional / masculine colouring79
Neutral plain	かもしれない	dictionary citation form17
Polite	かもしれません	polite spoken register78
Literary / archaic	かもしれぬ	appears in novels and historical dramas for character development10

「明日あした雨あめかも。」⁹
"Might rain tomorrow."

Register ladder for ～にちがいない

にちがいない has a shorter ladder and a tighter register footprint. The neutral plain rung itself is already on the formal side compared with かもしれない's neutral rung.¹¹

Rung	Form	Register
Neutral plain	にちがいない / に違いない	conveys strong personal conviction; skews toward writing11
Polite	にちがいありません	used when politer phrasing is wanted11
Formal written	に違いありません	the polite form written with kanji; common in formal writing and journalism11
Literary alternant	に相違ない	fits "writing or official contexts" where plain にちがいない would feel under-leveled11

Overall, にちがいない is meaningfully less common in casual spoken Japanese than かもしれない is. Use it when committing to a strong inference, not for everyday hedging.¹¹

Etymology aside: where the words come from

かもしれない breaks down as か (interrogative / "whether" coordinator) + も (inclusive particle, "even / also") + 知れない (negative potential non-past of 知る "to know"). Wiktionary's etymology summarises the literal meaning as "can not even know whether [preceding statement]."¹³ Bunpro restates this as "even so, we cannot know."⁷

The intuition is structural: the speaker cannot even rule out the possibility. That is why the marker stays neutral about whether the proposition is true or false. It asserts only that the negative cannot be excluded.

にちがいない breaks down as に (case particle) + 違い ("difference, discrepancy, mistake," the renyōkei nominal of 違う) + ない (negative-existence adjective). Bunpro notes explicitly that 違いない is "a combination of a noun and an い-Adjective, and not the negative form of the verb 違う."¹¹ The literal sense is "there is no discrepancy (with this being the case)," hence "no doubt."

Nuance and usage contexts

～かもしれない: floating a possibility

The core function is a tentative guess, hedged opinion, or softened self-assertion. DBJG glosses it as expressing possibility about a proposition the speaker is uncertain of.¹

かもしれない is often introduced by an epistemic adverb, such as もしかしたら, ひょっとしたら, or もしかして. Bunpro notes the pattern explicitly: "Due to もしかしたら presenting hypothetical situations, かもしれない will regularly appear in the same sentence (although it is not required)."¹⁴

もしかしたら今晩こんばん雪ゆきになるかもしれない。¹⁴
"Perhaps it will snow tonight."

もしかしたら採用さいようされるかもしれません。¹⁴
"I might be hired."

～にちがいない: declaring near-certainty from your own evidence

The core function is a strong subjective inference, typically built from circumstantial evidence the speaker has just noticed. Squires characterizes the conviction as a speaker stance toward the proposition rather than a report of external evidence.¹⁵ Niwasaburoo glosses the meaning as close to きっと～である, compatible with intensifying adverbs like 必ず and 絶対に.⁴

あの人ひと、昨日きのう会あった人ひとにちがいない。同おなじ服ふくを着きている。⁵
"That person must be the one I met yesterday. They're wearing the same clothes."

The example shows the typical shape: a piece of evidence the speaker has just noticed (the matching clothes) licenses the conviction.

How にちがいない differs from はず and だろう

はず, だろう, and にちがいない all sit above bare possibility, but each encodes a different mental move.

はず marks a logical expectation derived from a stated or assumed premise: "based on what I know, it should be so." Niwasaburoo describes はず as a conclusion based on logical reasoning. He also notes that the speaker can hold onto the はず judgment even when reality contradicts it.⁴

だろう is a softer conjecture or solidarity-seeking confirmation; LTL places it in the medium-certainty band, glossing it as "probably."⁶

にちがいない is a stronger subjective conviction, often built from circumstantial evidence the speaker has just noticed. LTL contrasts はず ("through objective analysis") with にちがいない ("through subjective thinking / gut feelings").⁶ In Niwasaburoo's frame, にちがいない still allows residual doubt and is therefore weaker than a bare assertion (である / だ), but the speaker has committed to one outcome.⁴

How かもしれない differs from でしょう and そう/らしい

かもしれない explicitly opens the door to the negative possibility: it might be so, or it might not. DBJG glosses possibility, not probability. The marker says nothing about which side is likelier.¹

でしょう / だろう is a positive-leaning guess in the medium-certainty band. It expresses "probably" or "right?" with a fair amount of certainty.⁶

そうだ, らしい, ようだ, and みたい are evidential markers in Aoki's typology. They encode the source of the inference, such as visual appearance, hearsay, or reasoned indirect evidence.³ かもしれない carries no such evidential commitment. It asserts pure possibility without telling the listener where the speaker's hunch came from.

Adverb pairing reference

Each end of the scale has its own family of epistemic adverbs. The pairings below are the ones found together in the cited sources.

Side of the scale	Adverb	Pairs with
Possibility	もしかしたら	かもしれない / かもしれません14
Possibility	ひょっとしたら	かもしれない14
Possibility	もしかして	かもしれない14
Certainty	きっと	にちがいない / だろう4
Certainty	必ず	にちがいない4
Certainty	絶対に	にちがいない4

Other pairings often listed in teaching materials, such as ことによると with かもしれない or おそらく and 間違いなく with にちがいない, are widely circulated. However, they were not found together in the sources surveyed here.

Negation and past on the embedded predicate

Neither marker conjugates for tense or negation in standard modern use. Negation and tense stay on the predicate they attach to.¹²⁷¹¹

行いかないかもしれない。⁷
"(I) might not go."

行いかなかったかもしれない。¹
"(I) might not have gone."

知しらなかったにちがいない。²
"(He) must not have known."

Good to know

Inserting だ before かもしれない or にちがいない after a noun or な-adjective

Both markers attach directly to a bare noun or な-adjective stem, so the copula だ drops. Bunpro states the rule for かもしれない ("The だ copula drops before かもしれない"⁷) and for にちがいない ("attaches to plain forms… of nouns (without だ)"¹¹). Maggie Sensei demonstrates: 犬かもしれません, not 犬だかもしれません.⁹

The correct shape is the bare noun or stem followed by the marker:

学生がくせいかもしれない。⁷
"(They) might be a student."

元気げんきにちがいない。¹¹
"(She) must be in good health."

かも is wrong for classroom prose, JLPT answers, and workplace email

かも is firmly casual-spoken and texting register. Maggie tags it "very casual"⁹ and Bunpro lists it as colloquial.⁷ In any context that calls for plain or polite written Japanese, write out かもしれない or use the polite かもしれません.⁷⁸ The mistake is common because anime, manga, and messaging apps lean heavily on the contraction. The form itself is correct, just tied to a specific register.

にちがいない is not for hearsay

にちがいない is the speaker's own conviction reached by inference. It is an epistemic, not evidential, marker in Aoki's typology.³ Hearsay ("I heard Tanaka is getting married") belongs to そうだ or らしい, not to にちがいない. This error is widespread because the English translation "must be" can lead learners to use にちがいない when reporting third-party information.

The corrected hearsay shape uses そうだ on the same predicate:

田中たなかさんは結婚けっこんするそうだ。³
"I hear Tanaka is getting married."

に違いない vs 間違いない

These adjacent forms share the etymological root 違い ("difference, mistake") but sit in different slots. に違いない is a sentence-final modal suffix attaching to a predicate (彼は学生に違いない). 間違いない is a standalone assertion ("これで間違いない," roughly "this is correct / no mistake"). Confusion is common because both translate as "no doubt" in English. The structural test is whether the form attaches to a preceding predicate (にちがいない) or stands as its own predicate (間違いない).

Kamo opens the door, chigai closes the case

The mnemonic maps onto the etymology of each marker, not just the gloss. かもしれない leaves both outcomes possible, which the literal "cannot even know whether"¹³ supports directly. にちがいない rules the alternative out, literally "there is no discrepancy."¹¹ Reading each form through its etymology gives a more durable mental anchor than the percentage scale.

See also

• ～でしょう / ～だろう: Conjecture and Confirmation in Japanese
• ～はず: How to Express Logical Expectation in Japanese
• Inferential Suffixes in Japanese: ～そう, ～よう, ～らしい, ～みたい Compared
• ～わけだ: How to Say "That's Why / It Follows That" in Japanese (Logical Conclusion)
• ～そうだ (Hearsay): How to Say "I Heard That" in Japanese
• ～らしい (Evidential): "Seems" and "Apparently" in Japanese

References

Footnotes

1. Makino, Seiichi and Michio Tsutsui. A Dictionary of Basic Japanese Grammar. The Japan Times, 1986. Entry "kamoshirenai," pp. 173–175. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

2. Makino, Seiichi and Michio Tsutsui. A Dictionary of Basic Japanese Grammar. The Japan Times, 1986. Entry "ni chigainai," pp. 308–310. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

3. Aoki, Haruo. "Evidentials in Japanese." In Wallace L. Chafe and Johanna Nichols (eds.), Evidentiality: The Linguistic Coding of Epistemology. Ablex, 1986, pp. 223–238. ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

4. Niwasaburoo (庭三郎). "39. 断定・確信." 現代日本語文法概説. https://niwasaburoo.amebaownd.com/posts/5757267/ ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

5. Kotoba-Note. "「かもしれない」「だろう」「にちがいない」の違いと使い分け." https://kotoba-note.com/kamoshirenai-darou-nichigainai-tsukaiwake ↩ ↩² ↩³

6. LTL Mandarin School. "Expressing Certainty in Japanese: Levels of Certainty Explained." https://ltl-japanese.com/grammar-bank/certainty-in-japanese/ ↩ ↩² ↩³ ↩⁴

7. Bunpro. "かもしれない – Japanese Grammar Explained." https://bunpro.jp/grammar_points/%E3%81%8B%E3%82%82%E3%81%97%E3%82%8C%E3%81%AA%E3%81%84 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹

8. JLPT Sensei. "JLPT N4 Grammar: かもしれない (kamo shirenai) Meaning." https://jlptsensei.com/learn-japanese-grammar/%E3%81%8B%E3%82%82%E3%81%97%E3%82%8C%E3%81%AA%E3%81%84-kamo-shirenai-meaning/ ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

9. Maggie Sensei. "How to use かもしれない / かも = kamo shirenai / kamo." https://maggiesensei.com/2018/06/09/how-to-use-%E3%81%8B%E3%82%82%E3%81%97%E3%82%8C%E3%81%AA%E3%81%84%E3%81%8B%E3%82%82-kamo-shirenai-kamo/ ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

10. Tofugu. "〜かもしれない for Uncertainty." https://www.tofugu.com/japanese-grammar/kamoshirenai/ ↩ ↩²

11. Bunpro. "に違いない – Japanese Grammar Explained." https://bunpro.jp/grammar_points/%E3%81%AB%E9%81%95%E3%81%84%E3%81%AA%E3%81%84 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰ ↩²¹ ↩²² ↩²³ ↩²⁴

12. Learn Japanese A-Z. "JLPT N3 Grammar: にちがいない (ni chigainai)." https://learnjapaneseaz.com/ni-chigainai.html ↩ ↩²

13. Wiktionary contributors. "かも知れない." Wiktionary, the free dictionary. https://en.wiktionary.org/wiki/%E3%81%8B%E3%82%82%E7%9F%A5%E3%82%8C%E3%81%AA%E3%81%84 ↩ ↩²

14. Bunpro. "もしかしたら." https://bunpro.jp/grammar_points/%E3%82%82%E3%81%97%E3%81%8B%E3%81%97%E3%81%9F%E3%82%89 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

15. Squires, Lauren. "The Interactional Consequences of Epistemic Indexicality: Some Thoughts on the Epistemic Marker -kamoshirenai." In Barbara Pizziconi and Mika Kizu (eds.), Japanese Modality: Exploring its Scope and Interpretation. Palgrave Macmillan, 2009. https://link.springer.com/chapter/10.1057/9780230245754_11 ↩