Lesson notes · ≈ JLPT N3 · Intermediate 1
〜ないことにします~nai koto ni shimasu
decide not to do
Use 「〜ないことにします」 to say you have personally decided not to do something.
How it works
Look at her pushing the snacks aside. She has decided for herself not to eat them in the middle of the night, and uses this pattern to express her decision.
Put the verb in the ない form, then add 「ことにします」. 食べる becomes 食べない, and する becomes しない.
「食べない」 simply says that you will not eat. Adding 「ことにします」 emphasizes that this is your own decision.
In the snack example, 「夜中に」 limits the time: the decision is not to eat snacks in the middle of the night, not to give them up at all times.
