In a previous article, we learned なる: something naturally becomes a certain way. Now let's meet its partner する: someone makes something a certain way.
In short:
| Direction | Meaning | |
|---|---|---|
| なる | Natural change (intransitive) | become... |
| する | Deliberate change (transitive) | make... / turn into... |
Basic Rules
Just like with なる, the way you attach する depends on the word type:
| Word type | Pattern | Example |
|---|---|---|
| i-adjective | い → くする | 大きい → 大きくする (make big) |
| na-adjective | な → にする | きれい → きれいにする (make clean) |
| Noun | noun + にする | 先生 → 先生にする (make someone a teacher) |
i-adjective + くする
Drop い from the i-adjective, add くする:
声を大きくする。 → Make the voice louder.
部屋を暖かくする。 → Make the room warmer.
テレビの音を小さくする。 → Turn the TV volume down.
Notice: since someone is deliberately changing something, the sentence always has を — "make what into what." This is the hallmark of a transitive verb.
na-adjective + にする
Drop な from the na-adjective, add にする:
部屋をきれいにする。 → Make the room clean.
外国人に親切にする。 → Be kind to foreigners.
子供を静かにする。 → Make the children quiet.
Noun + にする
息子を医者にする。 → Make one's son a doctor.
会議を来週にする。 → Set the meeting for next week.
なる vs する: Compare and It All Makes Sense
Same situation, but whether you use なる or する depends on who is doing the changing:
| なる (natural change) | する (deliberate change) |
|---|---|
| 部屋がきれいになった | 部屋をきれいにした |
| The room became clean | (I) made the room clean |
| 声が大きくなった | 声を大きくした |
| The voice got louder | (I) made the voice louder |
| 弟は医者になった | 弟を医者にした |
| My brother became a doctor | (Father) made my brother a doctor |
The pattern:
- なる → subject marked with が, describing "something changed on its own"
- する → object marked with を, describing "someone changed something"
Watch the Particles
This is the easiest place to make mistakes. Look carefully:
| なる | する | |
|---|---|---|
| Particle | が | を |
| Example | 部屋がきれいになった | 部屋をきれいにした |
| Reason | Intransitive: 部屋 is the subject | Transitive: 部屋 is the object |
Why? Because なる is intransitive — "the room became clean on its own" — so the room is the subject and takes が. Meanwhile する is transitive — "I made the room clean" — so the room is the object and takes を.
Another Use of にする: Making Choices
Besides "cause to become," にする has another extremely common everyday meaning — choosing/deciding:
コーヒーにします。 → I'll have coffee. (when ordering)
どれにしますか。 → Which one will you choose?
やっぱりこれにする。 → I'll go with this one after all.
You may have learned this back in N5. The underlying logic is actually the same as "cause to become" — you're setting the result to be a certain thing.
Summary
- i-adjective → くする; na-adjective / noun → にする
- する means "deliberately cause something to become a certain state" (transitive)
- なる means "naturally become a certain state" (intransitive)
- With する, the thing being changed takes を; with なる, the thing that changes takes が
- にする can also mean "choose/decide on"
Practice Questions
Q1. How do you say "make the room darker" in Japanese? (暗い = dark)
Show Answer
部屋を暗くする。 暗い is an i-adjective — drop い, add くする. The room is the thing being changed, so use を.
Q2. What's the difference between 「子供が静かになった」 and 「子供を静かにした」?
Show Answer
- 子供が静かになった → The children became quiet (natural change — maybe they calmed down on their own)
- 子供を静かにした → (Someone) made the children quiet (deliberate action — e.g. the teacher quieted them down)
なる describes natural change; する describes deliberate causation.
Q3. Which sentence is correct?
A. 部屋がきれいにした。 B. 部屋をきれいにした。
Show Answer
B is correct. する is transitive, so the thing being changed takes を, not が. 「部屋をきれいにした」= (I) made the room clean.