In short

To turn a whole clause into a noun - English “reading books”, “to speak Japanese”, “the fact that he came” - put the clause in the plain form and add こと (koto) or の (no). The clause then takes particles like any noun: 本を読むのが好きです (hon o yomu no ga suki desu, I like reading books). Choosing between こと and の is the whole lesson: の for what you can see and hear happening, こと for ideas, facts and abilities.

The rule

Japanese has no infinitive and no gerund. It has one move instead: stick a nominalizer on the end of a plain clause and the clause becomes a noun.

[ Clause in plain form ] + こと／の -> a noun phrase

This is the mirror image of lesson 18. There, a plain clause modified a real noun (私が読んだ本). Here it modifies こと or の, two empty nouns that contribute no meaning of their own - they exist only to make the clause grammatical as a noun. Written in kana, never as 事.

Step 1 - Put the clause in the plain form. As in every subordinate structure, politeness lives on the final verb of the sentence only. ×話しますことができます is wrong; 話すことができます is right, and the sentence is still polite because ます sits on できます at the very end.

Step 2 - Attach こと or の. Verbs and い-adjectives attach directly (読むこと, 忙しいこと). A な-adjective keeps its な (元気なこと), exactly as it does before a real noun. A noun predicate needs である or だという in front of こと: 彼が学生であること (kare ga gakusei de aru koto, the fact that he is a student). In front of の the copula だ simply surfaces as its attributive な, exactly like a な-adjective: 彼が学生なのを知りませんでした (kare ga gakusei na no o shirimasen deshita, I didn’t know he was a student).

Step 3 - Give the new noun a particle. The nominalized clause is now an ordinary noun phrase and takes が, を, は and the rest: 読むことが好きです, 来るのを待ちます, 私が言いたいのは……

Step 4 - Choose こと or の. This is the real work.

Use の for direct perception and things happening in front of you. After 見る (miru, see), 見える (mieru, be visible), 聞く (kiku, hear), 聞こえる (kikoeru, be audible), 待つ (matsu, wait for), 手伝う (tetsudau, help with) and 止める (tomeru, stop someone from doing), the clause names a concrete event you are witnessing or physically involved in. こと would wrongly turn it into an abstract fact:

• 子供が遊んでいるのを見ました。 - I saw the children playing (I watched it happen).

This holds for 見る and 聞く only in their literal senses - eyes and ears. 聞く meaning be told that… reports a proposition, not a sound, and takes こと／ということ (〜ということを聞きました, see below).

Use こと for abstract, reported or non-immediate predicates. After できる (dekiru, be able), 決める (kimeru, decide), 約束する (yakusoku suru, promise), 意味する (imi suru, mean), and before 大切だ (taisetsu da, important), 必要だ (hitsuyō da, necessary), the clause names an idea, a plan or a proposition - nobody is watching anything. With できる, 約束する and 意味する の is not available at all; with 決める and with 大切だ／必要だ こと is the standard choice, but の does occur, especially in speech.

Either works with the taste-and-skill adjectives: 好き (suki), 嫌い (kirai), 上手 (jōzu), 苦手 (nigate), 難しい (muzukashii), 楽しい (tanoshii). In speech の is the everyday choice; こと sounds more formal and written.

Step 5 - の is blocked directly in front of です／だ. When the nominalized clause is itself the predicate of a です／だ sentence, you must use こと: 趣味は写真を撮ることです. 撮るのです is not ungrammatical - it is simply read as the explanatory 〜んです of lesson 20, which says something else entirely.

The cleft: 〜のは〜だ. To put one element in focus - English “what I want to say is…”, “it was Tanaka who came” - nominalize everything else with の, mark it with は, and put the focused part after です:

• 私が言いたいのはそのことです。 (watashi ga iitai no wa sono koto desu) - What I want to say is that.
• 昨日来たのは田中さんです。 (kinō kita no wa Tanaka-san desu) - It was Mr Tanaka who came yesterday.

Note that inside the の-clause the subject is が, never は - the sentence’s single は has been spent on the nominalized chunk.

〜のが好き／上手／苦手 is the standard frame for talking about likes and skills, and the particle after の is が, not を - because 好き is an adjective, not a verb. This is the single most common particle error in the pattern.

ことができる is a fixed potential pattern: V-dictionary + ことができます (the dictionary form specifically, not any plain form). The の version is impossible. It is the formal, written alternative to the potential verb form, which gets its own treatment in Potential form and 〜ことができる.

ということ packages a proposition. When you nominalize something said, heard or known rather than done, wrap it in という first - it is the quotative と of lesson 25 plus こと: 彼が来ないということを聞きました (I heard that he isn’t coming). Plain 彼が来ないことを聞きました is possible but blander; ということ makes the “that…” clause audible as a report.

One warning. こと also appears in the set patterns 〜ことにする, 〜ことになる and 〜たことがある. Those are frozen idioms with meanings of their own (deciding, being decided, past experience), not the free nominalizer taught here. They belong to Experience with 〜たことがある and Formal nouns こと, もの, ところ in set patterns.

Forms & patterns

What attaches to what. The clause is plain; only な-adjectives and noun predicates need a linker.

Choosing こと or の.

Three frames worth memorizing whole.

Examples

Common mistakes

• 日本語を話すのができます。 (Nihongo o hanasu no ga dekimasu.) -> 日本語を話すことができます。 (Nihongo o hanasu koto ga dekimasu.) - ability is abstract, and ことができる is a fixed pattern. の never appears in it.
• 子供が遊んでいることを見ました。 (Kodomo ga asonde iru koto o mimashita.) -> 子供が遊んでいるのを見ました。 (Kodomo ga asonde iru no o mimashita.) - verbs of direct perception take の. こと would mean you learned the fact, not that you watched it.
• 本を読むのを好きです。 (Hon o yomu no o suki desu.) -> 本を読むのが好きです。 (Hon o yomu no ga suki desu.) - 好き is an adjective, so the thing liked is marked が. English “like” is a verb and pulls learners towards を.
• 日本語を話しますことができます。 (Nihongo o hanashimasu koto ga dekimasu.) -> 日本語を話すことができます。 (Nihongo o hanasu koto ga dekimasu.) - the clause before こと is always plain. The politeness of the sentence is carried once, by できます at the end.
• 私の趣味は写真を撮るのです。 (Watashi no shumi wa shashin o toru no desu.) -> 私の趣味は写真を撮ることです。 (Watashi no shumi wa shashin o toru koto desu.) - when the nominalized clause is what sits in front of です, use こと; 〜のです is heard as the explanatory 〜んです and changes the meaning.

Tips

• Two-word test: “watch” or “idea”. If you could put watch, hear, wait for, help with in front of the English, use の. If the clause names an idea, plan, rule or fact - something you could write on a list rather than film - use こと. Everything else (likes, skills, difficulty) takes either, and の is the safer default in conversation.
• こと sounds written, の sounds spoken. Both are correct with 好き and 難しい, but a Japanese speaker chatting says 読むのが好き and a textbook prints 読むことが好き. Copying the の habit will make your speech sound noticeably more natural.
• Avoid の pile-ups. 読むのが好きなのはこの本です is grammatical but clumsy, and a nominalizer の followed straight by another の (×読むのの……) is simply wrong. When a sentence needs a second nominalizer, switch one of them to こと.
• Do not read every こと as this lesson’s こと. 〜ことにする, 〜ことになる and 〜たことがある are frozen expressions that happen to contain the same word. If こと is followed by にする, になる or がある, you are looking at an idiom, not a nominalized clause.