This grammar point is very simple. You use くなる when the condition of something becomes a certain way by itself. On the other hand, くする is used when the condition of something is changed by an outside agent (either a person or a thing). In this post, let’s look at these two very basic constructions and how the Japanese works.
The grammar is very simple. First, you take an adjective or a noun and change them to their adverbial forms. For example, the adjective 長い has an adverbial form of 長く. The noun ひま has an adverbial form of ひまに.
After you make the adverbial forms of the adjective or the noun, you just put it before either なる or する. That’s it!
We talked about the く connector in this post and about the adverbial に in this post.
I should point out that while this grammar point appears in most JLPT books and websites as くなる and くする, as you can see in the picture above, nouns don’t use く! For that reason, I choose to call this point adverb ✙ なる and adverb ✙ する.
Here are some examples with なる:
① 熱が下がって、気分がだいぶ{よくなりました}。
=fever will go down and then feeling became considerably better
= My fever went down and then I felt much better.
② この仕事が終わったら、少し{ひまになる}と思います。
= when work finishes, a little bit, will become free, I think
= I think that when work finishes, I’ll have more time.
③ このごろ仕事が減って、前ほど{忙しくなくなった}。
= these days, work decreased and so as much as before, became not busy
= These days, I have less work and so I’m not as busy as before.
④ きみは{大人になったら}、{何になりたい}の。
= when you become an adult, what want to become
= When you grow up, what do you want to be (and explain)?
In example 1, the sense is that when the fever went down, the person’s mood got better by itself.
In example 2, the condition of having more free time arises naturally when work decreases. (ひま can have the connotation of having absolutely nothing to do and can be considered rude by some people. I use the word for myself sometimes, but I never seriously refer to other people as ひま.)
Concerning example 3, the negative form of 忙しい is with 忙しくない. If you want to make THAT into an adverb, it will become 忙しくなく. This was very difficult for me when I was just beginning Japanese.
Finally, Example 4 shows that the question word of 何 can be treated as a noun. This makes sense if you think of it as a placeholder for whatever answer the listener will give. Also, because the speaker uses the word きみ we know that the speaker and listener are close. It would make sense if it were a parent-child relationship. きみ is NOT used with people that you have just met or that you don’t know well!
Here are some examples with する:
⑤(父が子どもに)もっと部屋を{きれいにしなさい}。
= father to his child: a little bit more the room, make it clean
= Clean up your room a bit more.
⑥ このケーキ、ちょっと大きいから、{半分にして}ください。
= this cake, a bit big and so make it half please
= This cake is a bit (too) big so please cut it in half.
⑦ スカートを5センチぐらい{短かくして}ください。
= this skirt, about 5 centimeters make it short please
= Please shorten this skirt about 5 centimeters.
Notice that examples 5, 6 and 7 all include someone (other than the speaker) making a thing (a room, a cake, and a skirt) a different condition than the current one. This is when you will want to use adverb + する.
The grammar points of adverb ✙ なる and adverb ✙ する are pretty simple to understand. That is why they are considered Level N5. Adverb ✙ なる shows a person or a thing becoming a different condition by itself. Adverb ✙ する shows a person or a thing changing to a different condition by a different person or thing.
Thanks for reading, and see you next time!