Japanese Complement Clauses with こと: The Abstract Nominalizer for Sentences-as-Nouns
Japanese complement clauses with こと turn a whole sentence into a noun. That lets an entire clause fill the subject or object slot of a larger sentence.1 If you can already build relative clauses, this is the next step: embedding a full statement where a single noun would normally go.
Overview
こと (kanji 事) is a noun that basically means "thing." As a grammatical tool, it is the abstract nominalizer: it takes a clause and packages it as a noun.21 That noun can then take a case particle and behave like any other noun in the sentence.34
こと as the abstract nominalizer
The noun 事 is "a versatile word for intangible, abstract things," covering ideas, events, and experiences; it "refers to 'something intangible.' Sometimes it's what you say, sometimes it's what you think, sometimes it's what you do, and sometimes it's just what occurs."2 That abstract core carries straight into its grammatical job.
As a nominalizer, こと "is a noun that is often translated as 'thing,'" and so "literally means 'the thing that is (A)' when used for nominalization."1 Attach it to a verb in dictionary form, and the verb, along with the clause it heads, becomes a noun: 食べる ("to eat") becomes 食べること ("eating").4
Once a clause is nominalized this way, it can take a case particle (が / を / は) and fill a subject or object slot.34 For an abstract or general activity, こと is the default nominalizer.
運動をする事は、大切だ。2
"It is important to exercise."
本を読むことが好きです。4
"I like to read books."
Complement clause vs relative clause
A relative clause and a complement clause both embed one clause inside another, but they do different jobs. A relative clause modifies a noun that sits outside it, as covered in Japanese Relative Clauses: Modifying a Noun With a Whole Sentence. A こと complement clause is different: the clause itself becomes the noun.
In 運動をする事は、大切だ, the string する事 is not modifying a separate noun. The whole clause "doing exercise" has been turned into the noun argument that the topic marker は attaches to.2 The clause is the subject, not a description attached to one.
Ask what the noun is. If こと names a separate thing the clause describes, you are looking at a modified noun. If こと is the clause itself, repackaged as a noun argument that takes が / を / は, you have a complement clause.
For the wider family of embedded clauses and how each type fits together, see Japanese Subordinate Clauses: How Embedded Clauses Work (Relative, Complement, Quotation, Embedded Question).
Form: how to attach こと
The mechanics are simple: use a clause in plain form, then add こと. Tense, politeness within the clause, and negation are all settled inside the clause before こと is added.
Plain-form clause + こと
The clause before こと ends in plain (dictionary or short) form. To nominalize, "just add it to the end of a verb in its dictionary form."4 Tense and negation live inside the clause, before こと, never on こと itself.
私はゲームをすることが大好きです。4
"I love playing games."
Negation works the same way: the verb takes its plain negative form, then こと attaches. The plain negative 喋らない sits directly before こと.
大きい声で喋らないこと。1
"Not talking loudly."
What the resulting noun does in the sentence
Once nominalized, the clause-noun behaves like any noun. It takes が when it is the subject of a predicate adjective or verb, を when it is the object, は when it is topicalized, and です / だ when it is the predicate nominal itself.34
歌う（こと・の）が好き（だ・です）。3
"I like singing."
走る（こと・の）は私の趣味（だ・です）。3
"Running is my hobby."
A single sentence can hold more than one こと-clause in different roles. One may be topicalized with は, while another stands as the です predicate.4
私の趣味は料理を作ることです。4
"My hobby is cooking."
こと vs の: which nominalizer
Japanese has two general-purpose nominalizers, こと and の. Much of learning こと is learning when の is the better choice, or the only choice. The の half of this pair is treated in full in The の Particle: Possessive, Nominalizer, Attributive.
The default split: abstract/general こと vs concrete/perceived の
The reference rule contrasts general and specific meaning: "the nominalizer の is used to refer to a specific concrete event, whether it has happened or might happen in the future, while こと is used generally."5 Put for learners, こと "is used for intangible or conceptual ideas, while の is used for more tangible or physical concepts."4
Many subject and object slots accept either nominalizer with no change of meaning.6 Both 走るのが好きです and 走ることが好きです mean "I like running," with の "feeling more physical" and こと "more conceptual."4
The following both allow either nominalizer in the parenthesized slot.
ブログを読む（こと・の）が好きです。6
"I like reading blogs."
彼が来る（こと・の）を知っている。6
"I know he is coming."
When only こと works
A specific set of matrix predicates, the main predicates that take the embedded clause, demands こと and rejects の. These include the copula predicate-nominal frame 「Xは～ことだ」, ability できる, experiential or occurrence ある, the decision idioms にする / になる, and a group of communication and internal-thought verbs such as 考える, 頼む, and 命じる.5
The cleanest test is the predicate-nominal ～ことだ frame: only こと can sit before だ / です as the predicate noun, the noun-like part of the predicate.3
私の趣味は料理を作ることです。4
"My hobby is cooking."
Ability with できる is the other clear こと-only slot.
日本語で歌う事ができます。2
"I can sing in Japanese."
When only の works
The mirror image is a set of matrix verbs that demand の and reject こと. These are the direct-perception verbs 見る, 見える, 聞く, 聞こえる, 感じる, plus "stop / wait for / help / interrupt" verbs such as 止める, 待つ, 手伝う, and 邪魔する.56 Each of these takes a concrete event that is directly perceived or directly acted on. That is exactly the slot reserved for の as the concrete complement-clause nominalizer.5
むすめがピアノをひいているのを見た。6
"I watched my daughter play the piano."
ドアが開くのを待ってください。6
"Please wait for the door to open."
The の-only rule for 見る, 聞く, 感じる, 待つ, and 止める sits just above N4. Treat it as a boundary to recognize rather than master at this stage; using こと in these slots is ungrammatical, not merely unnatural.
The same restriction holds for 感じる.
元気がでるのをかんじる。6
"I feel my energy appear."
The こと idiom family
Several high-frequency patterns are built on こと plus a fixed predicate. Each has its own dedicated article. This section maps the family so the pieces are recognizable, then points you to the deeper treatment of each.
Experience, ability, and decision at a glance
Four patterns dominate the family. ～ことができる expresses potential or ability ("can do"). ～ことがある covers experience ("have done X before") and occurrence ("there are times when"). ～ことにする marks a speaker-internal decision. ～ことになる marks something externally decided or something that simply comes about.78910
For ability, ～ことができる is the こと-only potential frame.
私の姉は歌う事が出来る。7
"My older sister can sing."
The full treatment, including the contrast with the short potential form, lives in ～ことができる: How to Say "Can Do" in Japanese.
～ことがある carries two senses. The occurrence sense reads "there are times when."
頑張ってもうまくいかない事がある。8
"There are times when no matter how hard you try, it doesn't go well."
The experiential ～たことがある sense ("have done X before") is handled in ～ことがある: How to Say "I Have Done X Before" in Japanese.
The decision pair splits on who decides. ～ことにする marks something the speaker decided by their own choice.9
九時に終わるか分からなかったが、早めに帰る事にしました。9
"I didn't know if it would finish at nine, but I decided to go home early."
～ことになる marks something decided or brought about by external factors, an outcome arrived at without the speaker's direct choice.10
来月転勤することになった。10
"It's been decided that I'll be transferred next month."
Both decision patterns are treated together in ～ことにする / ～ことになる: Decide vs. It Was Decided.
Nuance and usage contexts
Beyond grammaticality, こと and の also differ in register, or level of formality. Learners can hear this difference once they know to listen for it.
Register: こと in written and formal Japanese
The register split tracks the abstract/concrete split. 事 is used for "broad, abstract concepts," while の "is used to talk about more concrete actions or something familiar to you," and "using the particle の here sounds more casual."2 In practice, conversational speech leans on の wherever either nominalizer is grammatical. Written and formal Japanese, definitions, and the ～ことだ frame favor こと.42
Rules and regulations use a terse imperative こと: a clause plus こと standing alone as a directive. The 事 may be written in kanji, but "it's also common to see it written in kana."2
友達には、意地悪しないこと。2
"Don't be mean to your friends."
This is the register behind notices and posted rules.
大きい声で喋らないこと。1
"No talking loudly."
Good to know
Why こと feels "abstract" and の feels "right here"
The fastest way to remember the こと-vs-の rule from meaning is to map each nominalizer to a feeling. 事 "refers to 'something intangible'" and names ideas, facts, and matters, while の attaches to "a specific concrete event."52 Read こと as "the matter or fact of," and の as "this very event I am seeing or hearing." That mapping alone predicts why perception verbs take の: you point の at the event in front of you.
こと is the noun 事 wearing a grammatical hat
こと is literally the noun 事 ("thing, matter, affair") grammaticalized into a nominalizer.21 Keeping that origin in mind explains two things at once: why a こと-clause still means "the thing that is (A)," and why こと can stand as a full predicate noun in the ～ことだ frame where の cannot.1
Forcing の into a copula, ability, or decision slot
A common error is reaching for の in a slot reserved for こと. Writing 私の趣味は料理を作るのです to mean "my hobby is cooking" does not deliver that predicate-nominal meaning. The ～ことだ frame, ability できる, and the decision idioms にする / になる are all こと-only.53 The correct form keeps こと.
私の趣味は料理を作ることです。4
"My hobby is cooking."
Using こと after a perception or waiting verb
The opposite error is putting こと after a direct-perception verb such as 見る, 聞く, or 感じる, or after a waiting or stopping verb such as 待つ or 止める. A sentence like 子供が走ることを見た is wrong. These matrix verbs take の only.56 The correct form uses の.
子供が走るのを見た。5
"I watched the child run."
Do not over-police the interchangeable slots
Many subject and object slots accept either nominalizer with no change of meaning, including 好き / 嫌い predicates, 知っている, and 忘れる. Both 走るのが好きです and 走ることが好きです are correct. の sounds more concrete and casual, while こと sounds more conceptual and formal.64 Beginners sometimes try to enforce a single "right" answer in these slots; there is none.
See also
- Japanese Subordinate Clauses: How Embedded Clauses Work (Relative, Complement, Quotation, Embedded Question)
- Japanese Relative Clauses: Modifying a Noun With a Whole Sentence
- Japanese Quotation with と: How to Say What Someone Said or Thought
- Potential Form: ～られる, ～える, できる
- The が Particle: Subject Marker (and More)
- The を Particle: Direct Object