の and こと both turn a clause into a noun, but some constructions require one of them. Directly perceived actions generally take の, as in 走るのを見た. Fixed patterns such as ことができる require こと. In other contexts both can work; the choice depends on the verb, construction, and intended viewpoint.
also written no / koto / のを見る / ことができる
の — the event, watched as it happens · こと — the fact, held at a distance · some verbs take only one of them
Sentences
6 sentences, word by word
の — watching it happen
plain past
。
Hover, tap, or tab through the words.
公園
こうえん ［0］
park
noun · the place the action happened in
Ask Rikai
で
で
in; at — where it happens
particle · where the action happens, or the manner it happens in
Ask Rikai
彼
かれ ［1］
he
pronoun · the subject inside the nominalised clause
Ask Rikai
が
が
marks the subject
particle · what the sentence is about — with 好き, 分かる and potentials, the object
Ask Rikai
走る
はしる ［2］
run
godan verb · plain form, which the nominaliser attaches to
Ask Rikai
の
の
the sight of it
nominaliser · compulsory after a verb of perception
のを見るN4see something happen
Ask Rikai
を
を
marks the direct object
particle · pronounced お
Ask Rikai
見た
みた
saw
plain past of 見る · a verb of direct perception
Ask Rikai
I saw him running in the park.
こと — the frozen potential
polite
。
Hover, tap, or tab through the words.
千
せん ［1］
thousand
numeral · combining with the counter that follows
Ask Rikai
メートル
メートル
metres
counter · loanword, always katakana
Ask Rikai
泳ぐ
およぐ ［2］
swim
godan verb · plain form before こと
Ask Rikai
こと
こと
the act of; the fact of
noun · turns the verb in front into a noun
Ask Rikai
が
が
marks the subject
particle · what the sentence is about — with 好き, 分かる and potentials, the object
Ask Rikai
できます
できます
can do
polite できる · ことができる is the long potential form
ことができるN4be able to
Ask Rikai
I can swim a thousand metres.
こと — defining what something is
polite
。
Hover, tap, or tab through the words.
趣味
しゅみ ［1］
hobby
noun · what you do for pleasure
Ask Rikai
は
は
as for — the topic
particle · written は, read わ
Ask Rikai
写真
しゃしん ［0］
photograph
noun · 写真を撮る is the set phrase
Ask Rikai
を
を
marks the direct object
particle · pronounced お
Ask Rikai
撮る
とる ［1］
take
godan verb · plain form before こと
Ask Rikai
こと
こと
the act of; the fact of
noun · turns the verb in front into a noun
Ask Rikai
です
です
(polite ending)
copula · a definition, which is why こと and not の
ことですN4is (defining)
Ask Rikai
My hobby is taking photographs.
の — hearing it happen
plain past
。
Hover, tap, or tab through the words.
隣
となり ［0］
next door
noun · the adjacent house or room
Ask Rikai
で
で
in; at — where it happens
particle · where the action happens, or the manner it happens in
Ask Rikai
子供
こども ［0］
child
noun · covers singular and plural
Ask Rikai
が
が
marks the subject
particle · what the sentence is about — with 好き, 分かる and potentials, the object
Ask Rikai
泣く
なく ［0］
cry
godan verb · 泣く for people, 鳴く for animals
Ask Rikai
の
の
the sound of it
nominaliser · compulsory after 聞く as a verb of perception
のを聞くN4hear something happen
Ask Rikai
を
を
marks the direct object
particle · pronounced お
Ask Rikai
聞いた
きいた
heard
plain past of 聞く · direct perception, not 'asked' here
Ask Rikai
I heard a child crying next door.
の — the everyday choice
polite
。
Hover, tap, or tab through the words.
本
ほん ［1］
book
noun · the object of the reading
Ask Rikai
を
を
marks the direct object
particle · pronounced お
Ask Rikai
読む
よむ ［1］
read
godan verb · plain form before the nominaliser
Ask Rikai
の
の
the doing of it
nominaliser · either works here; の sounds closer to the activity
のが好きN4like doing
Ask Rikai
が
が
marks the subject
particle · what the sentence is about — with 好き, 分かる and potentials, the object
Ask Rikai
好き
すき ［2］
like
な-adjective · takes が for what is liked
Ask Rikai
です
です
(polite ending)
copula · politeness on a な-adjective predicate
Ask Rikai
I like reading books.
こと — the fact, not the event
polite
。
Hover, tap, or tab through the words.
彼
かれ ［1］
he
pronoun · the subject of the nominalised clause
Ask Rikai
が
が
marks the subject
particle · what the sentence is about — with 好き, 分かる and potentials, the object
Ask Rikai
来る
くる ［1］
come
irregular verb · plain form before こと
Ask Rikai
こと
こと
the act of; the fact of
noun · turns the verb in front into a noun
Ask Rikai
を
を
marks the direct object
particle · pronounced お
Ask Rikai
忘れて
わすれて
forgot
て-form of 忘れる · linking into いました
Ask Rikai
いました
いました
had (forgotten)
polite ている in past · a state, not an action
ことを忘れるN4forget that
Ask Rikai
I'd forgotten that he was coming.
Structure
Where each one is forced
Clause＋の＋見る / 聞く
彼が走るのを見た. Use の to nominalise an action directly witnessed with 見る; こと cannot replace it in this construction.
Clause＋ことができる / ことがある / ことにする
泳ぐことができる. These are set phrases; の cannot be swapped in.
Clause＋ことです
趣味は写真を撮ることです. When the clause is the predicate of a definition, こと is the natural choice.
Clause＋の / こと＋が好き / が難しい
泳ぐのが好き, 泳ぐことが好き. Both work. の feels closer to the activity, こと more like the idea of it.
Meaning
Distance, and the three rules that override it
Viewpoint can help, but the construction comes first.の can present an event more directly, while こと can present it as a fact or idea. Specific verbs and set patterns often determine which nominalizer is natural.
Direct perception usually takes の. For an observed or heard action, say 子供が泣くのを聞いた. Replacing it with こと is unnatural for this event-perception reading.
Rule two: some patterns are frozen with こと.ことができる, ことがある, ことにする, ことになる. These are set phrases rather than free choices, so the question of nuance never arises.
Definitions commonly use こと.私の趣味は本を読むことです names the activity that constitutes the hobby. This preference is tied to the predicate construction, not a universal ban on の elsewhere.
In some other contexts, either works.泳ぐのが好き and 泳ぐことが好き are both fine. The first sounds like you enjoy the swimming; the second treats swimming more as an activity or concept. In speech, most people say の.
Watch out
Where の and こと go wrong
走ることを見た
For directly seeing an action happen, use の: 走るのを見た. This is a restriction of the direct-perception construction, not a rule about every use of 見る.
泳ぐのができる
The long potential is frozen with こと: 泳ぐことができる. The same applies to ことがある and ことにする.
Confusing the nominaliser の with explanatory のだ
読むのが好き nominalises. 読むのです explains — “the thing is, I read.” Same kana, different job, and it is why a definition uses こと.
Also asked
Questions people ask about の and こと
Is there a simple rule?
Perception of an event generally uses の; fixed expressions such as ことができる use こと. Elsewhere both may be possible, depending on the construction and viewpoint.
泳ぐのが好き or 泳ぐことが好き?
泳ぐのが好き and 泳ぐことが好き both work. The latter can sound more like a general activity or concept; it does not imply mere approval without enjoyment.
Why is it 走るのを見た?
When 見る means directly witnessing an action, the clause takes の. 走ることを見た does not express that same direct-perception meaning.
Is 泳げる the same as 泳ぐことができる?
Same meaning, different weight. The short potential 泳げる is the everyday form; 泳ぐことができる is longer, slightly more formal, and common in writing and announcements.
What is 〜たことがある?
The experience pattern — 日本に行ったことがある, I have been to Japan. It is frozen with こと, and the past form in front is what makes it about experience rather than habit.
How many different の are there?
Three worth separating. After a noun it links — 本の表紙. After a clause it nominalises — 走るのを見た. And after an adjective or clause it can stand in for a noun you did not say — 赤いのが好き, I like the red one. Only the middle one is the nominaliser this page is about.
References
Sources and usage notes
Japan Foundation: こと（1） — Compares clause nominalization with の and こと and lists constructions that restrict the choice.
