Grammar Info
N3 Lesson 3: 9/23

事(こと)にする
to decide on, have decided to

Structure
Casual
Formal
Verb[る]+ ことにする
Verb[ない]+ ことにする
Verb[る]+ ことにします
Verb[ない]+ ことにします
Details
Register Standard
使用域 一般
Information
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)はワクチン打(う)ったから明日(あした)は仕事(しごと)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)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)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.
Caution
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)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)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)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)食(た)べないことにしない
    I have decided to not not eat meat from now on.
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今日(きょう)はエイプリルフールだからウソしか言(い)わないことにする
Since today is April Fool's, I've decided to only tell lies.
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あの場所(ばしょ)で、記念(きねん)写真(しゃしん)撮(と)ることにします
We've decided to take the commemorative photo at that place.
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九(く)時(じ)終(お)わるか分(わ)からなかったが、早(はや)めに帰(かえ)ることにしました
I didn't know if it would finish at nine but, I decided to go home in advance.
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Books
Genki II 1st Edition
Page 227
Genki II 2nd Edition
Page 259
[DBJG] A Dictionary of Basic Japanese Grammar
Page 204
Tae Kim's Japanese Grammar Guide
Page 132
[AIAIJ] An Integrated Approach to Intermediate Japanese
Page 16