In a similar way to how
にする means to ‘decide on’ something,
ことにする means to decide on a situation, or course of action. This grammar point is a combination of 事(こと) ‘thing’ in its role as a nominalizer (a word that creates a noun-phrase), and
にする ‘to make it (A)’. Literally this grammar structure can be translated as ‘to do (A)’, or ‘to make it (A)’.
ことにする may be used with verb-phrases that are either in the affirmative, or negative (using ない) forms. As こと itself is a 名詞(めいし) (noun), the verb (or auxiliary verb) that comes before it will need to be in its 連体形(れんたいけい) (attributive form).
-
今日(きょう)はワクチンを打(う)ったから、明日(あした)は仕事(しごと)を休(やす)むことにする。
Today I got vaccinated, so I will go with taking the day off tomorrow.
タマキさんと箱根(はこね)に行(い)くことにする。
I have decided to go to Hakone with Tamaki-san.
今日(きょう)は雨(あめ)が降(ふ)っているから、明日(あした)行(い)くことにしない?
It is raining today, don't you want to make it that we go tomorrow?
勝手(かって)に僕(ぼく)が行(い)くという事(こと)にしないでください。
Please don't decide that I will go without asking me. (Please don't assume that I want to go)
にする is a strong expression, which tends to make the speaker sound like they are fairly resolute in their decision.
The
する in
にする itself may also be seen using ない, to mean that someone has decided ‘against’ doing something. Due to this, careful attention will need to be paid to the overall structure of the sentence.
-
これからは肉(にく)を食(た)べることにする。
I have decided to eat meat from now on.
これからは肉(にく)を食(た)べることにしない?
Let's decide to eat meat from now on.
これからは肉(にく)を食(た)べないことにする。
I have decided to not eat meat from now on.
これからは肉(にく)を食(た)べないことにしない。
I have decided to not not eat meat from now on.