Japatan
Formation
- V辞書形 + ことにする
- Vない形 + ことにする
Explanation
This expresses a decision the speaker makes by themselves. Koto ni shimasu is deciding now, koto ni shimashita is having decided, and koto ni shite imasu is a personal rule or habit decided earlier and still kept. It is not used for things you cannot decide, like the weather.
Examples
夏休みに 友だちと 京都へ 行くことに しました。
I decided to go to Kyoto with my friends during summer vacation.
体の ために、お酒を 飲まないことに しました。
For my health, I decided not to drink alcohol.
毎晩 寝る 前に 三十分 本を 読むことに して います。
I make it a rule to read for thirty minutes every night before bed.
会社の 近くに 引っ越すことに しました。
I decided to move near my office.
Notes
When someone else or an organization made the decision, use koto ni naru. In the meaning of deciding, koto ni suru follows verbs, not adjectives.
Related grammar
Mini quiz
1来年、アメリカの 大学に 留学（ ）。
Answer and explanation
Answer: 2 することに しました
'I've decided to study abroad at an American university next year.' The pattern is dictionary form + koto ni shimashita. It uses ni, not ga, and the dictionary form suru.
2A「ケーキ、食べないんですか。」
B「ええ、今月は 甘い ものを（ ）ことに したんです。」
Answer and explanation
Answer: 4 食べない
'Yes, I decided not to eat sweets this month.' B explains why she isn't eating cake, so tabenai koto ni shita fits. Taberu is the opposite meaning, and -nakute or -nakatta cannot come before koto ni suru.
3「ことに する」の 使い方が 正しい 文は どれですか。
Answer and explanation
Answer: 3 毎朝 一時間 歩くことに して います。
Sentence 3, 'I make it a rule to walk for an hour every morning', is correct. In 1, you cannot decide the weather. 2 uses an adjective and 4 attaches koto directly to a noun, so both are wrong.
