〜くする / 〜にする: Make It | Japanese Grammar

If 〜くなる / 〜になる is how a state comes about by itself, then 〜くする / 〜にする is how someone makes it come about on purpose. The building block is identical — take the adjective's adverbial form (い-adjective → 〜く, na-adjective/noun → 〜に) and add a verb — but the verb is now する, "to do / to make." The whole meaning of the sentence flips from "it changed" to "I changed it." English hides this difference inside one word: "the soup got warm" and "I warmed the soup" both use ordinary verbs and you infer agency from context. Japanese refuses to leave it to context. It forces you to pick なる or する, and picking correctly is one of the clearest tests of whether you are thinking about who is responsible for a change.

The core pattern: adverb + する

Just like なる, する attaches to the adverbial form, never to the plain adjective. You are saying you "do [something] [in a certain way]" — 大きく (biggly) + する (do) = "make it big." The 〜く / 〜に split is the adverbial form again, so everything you learned for なる carries straight over; only the verb changes.

Word type	Adverbial link	する	Meaning
い-adjective 大きい	大きく	大きくする	make it bigger
い-adjective 短い	短く	短くする	make it shorter
い-adjective 安い	安く	安くする	make it cheaper
な-adjective 静か	静かに	静かにする	make/keep it quiet
な-adjective きれい	きれいに	きれいにする	make it clean/tidy

い-adjectives: 〜くする

Replace the final 〜い with 〜く and add する. Because する is transitive, there is now an object, marked with を — the thing you are acting on.

音が小さいから、もう少し大きくして。

oto ga chiisai kara, mō sukoshi ōkiku shite

It's too quiet, so turn it up a little more.

前髪、もう少し短くしてもらえますか。

maegami, mō sukoshi mijikaku shite moraemasu ka

Could you make my bangs a bit shorter?

まとめて買うので、少し安くしてくれませんか。

matomete kau node, sukoshi yasuku shite kuremasen ka

I'm buying in bulk, so could you make it a little cheaper?

Notice how naturally these land in requests and service situations — turning the volume up, trimming hair, haggling a price. That is because 〜くする is inherently about someone deliberately adjusting something, which is exactly what you do when you ask a person to change it for you.

な-adjectives and nouns: 〜にする

na-adjectives drop な and add に; nouns add に directly. Then する does the work.

お客さんが来る前に、部屋をきれいにしておこう。

okyaku-san ga kuru mae ni, heya o kirei ni shite okō

Let's tidy up the room before the guests arrive.

髪を切って、少し大人っぽい感じにした。

kami o kitte, sukoshi otonappoi kanji ni shita

I got a haircut and gave myself a slightly more grown-up look.

The most famous 〜にする sentence in the language is a request to be quiet — and it shows する's "make it so" force perfectly. You are asking people to make themselves quiet.

映画が始まりますので、静かにしてください。

eiga ga hajimarimasu node, shizuka ni shite kudasai

The film is about to start, so please be quiet.

する vs なる: the whole point

Set the two verbs side by side and the contrast is total. Same adjective, same adverbial link, opposite responsibility:

なる (spontaneous)	する (deliberate)
部屋が静かになった The room went quiet.	部屋を静かにした I made the room quiet.
音が大きくなった The sound got louder.	音を大きくした I turned the sound up.
が marks the changing thing	を marks the thing you change

音を大きくしたら、隣の人に怒られた。

oto o ōkiku shitara, tonari no hito ni okorareta

When I turned the volume up, the person next door got angry at me.

💡

The single reliable test: is there someone doing it on purpose? If yes → する (with を). If it just happened → なる (with が). The weather, your mood, and the passage of time are almost never something you "do," so they take なる.

The transitive/intransitive parallel

This する/なる pair is not an isolated adjective quirk. It is the adjective-side reflection of one of Japanese's most systematic patterns: paired transitive and intransitive verbs. Just as ドアが開く (the door opens) contrasts with ドアを開ける (someone opens the door), and 温度が上がる (the temperature rises) contrasts with 温度を上げる (someone raises it), 静かになる (it becomes quiet) contrasts with 静かにする (someone makes it quiet). Learn to hear なる and する as "the intransitive one" and "the transitive one," and a huge swath of the verb system suddenly rhymes. The full paradigm lives on transitivity pairs.

エアコンをつけて、部屋を涼しくした。

eakon o tsukete, heya o suzushiku shita

I turned on the AC and made the room cooler.

When 〜にする attaches to a plain noun and there is no "making it be that way" involved, it takes on a second, idiomatic meaning: choosing / deciding on something. コーヒーにする means "I'll go with coffee" (deciding off a menu), not "make it into coffee." This selecting sense is common enough that it is treated separately — the closely related 〜ことにする covers deciding to do an action. For now, just be aware that 〜にする does double duty: "make it X" and "opt for X."

私はカレーにする。あなたは？

watashi wa karē ni suru. anata wa?

I'll have the curry. How about you?

Noun + にする = turn/appoint into

There is one more use worth knowing, and it pairs beautifully with the なる page. Where 医者になる means "become a doctor" (the person's own change), 〜医者にする means "make/turn someone into a doctor" — an agent assigns or transforms someone into that role. The intransitive/transitive contrast is identical to the adjective cases; only the second slot is a noun-role rather than a quality.

両親は私を医者にしたがっている。

ryōshin wa watashi o isha ni shitagatte iru

My parents want to make me (become) a doctor.

この部屋を子どもの勉強部屋にしよう。

kono heya o kodomo no benkyō-beya ni shiyō

Let's turn this room into the kids' study.

So the two pages together give you a matched set: なる for "X turns into Y on its own," する for "someone turns X into Y."

Common Mistakes

1. Swapping する and なる — the core error. If nobody is deliberately causing the change, you need なる, not する. Using する for the weather warming up implies a person warmed it.

❌ 春になって、暖かくした。

Incorrect — 'it got warm' is spontaneous, so する is wrong.

✅ 春になって、暖かくなった。

haru ni natte, atatakaku natta

Spring came and it got warm.

2. Forgetting the adverbial link. する needs an adverb; an い-adjective must become 〜く first.

❌ 部屋を明るいする。

Incorrect — する attached to the plain form 明るい.

✅ 部屋を明るくする。

heya o akaruku suru

I'll make the room brighter.

3. Keeping the な of a na-adjective. The adverbial form is 〜に, with な dropped.

❌ 部屋をきれいなにする。

Incorrect — な not dropped before に.

✅ 部屋をきれいにする。

heya o kirei ni suru

I'll make the room clean.

4. Marking the object with が instead of を. する is transitive, so the thing you change is the object (を), not the subject.

❌ 音が大きくする。

Incorrect — が where を is needed with transitive する.

✅ 音を大きくする。

oto o ōkiku suru

I'll turn the sound up.

5. Using を with なる by analogy. The mirror mistake: once you know 静かにする takes を, it is tempting to give 静かになる one too. Don't — なる stays intransitive.

❌ 子どもたちを静かになった。

Incorrect — なる is intransitive and cannot take を.

✅ 子どもたちが静かになった。

kodomo-tachi ga shizuka ni natta

The children became quiet.

Key Takeaways

する attaches to the adverbial form (〜く / 〜に), exactly like なる — only the verb differs.
する = deliberate, transitive. Someone acts on an object (marked を) to change it.
する vs なる is the adjective-side of the transitive/intransitive verb pairs. する is "the transitive one" (開ける, 上げる); なる is "the intransitive one" (開く, 上がる).
The reliable test is agency: is someone doing it on purpose? Yes → する; no → なる.
〜にする with a plain noun also means "decide on / I'll have" (コーヒーにする), a separate everyday use.

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