〜ことにする koto ni suru
〜ことにする（koto ni suru）means “decide to (do)”. Expresses a decision made by the speaker through their own will — 'I (have) decided to do…'. Formation: V-る／V-ない＋ことにする
decide to (do)
N4Formation
V-る／V-ない＋ことにする
Explanation
Expresses a decision made by the speaker through their own will — 'I (have) decided to do…'. Attaches to the dictionary form (to do) or ない form (decide not to do). The た-form ことにした reports a decision already made; ことにしている expresses an established habit or rule one keeps to.
Examples
Note
Contrast with 〜ことになる, which expresses a decision/outcome reached by circumstances or others (not one's own will).
FAQ
〜ことにする — what does it mean and how is it used?
decide to (do). Expresses a decision made by the speaker through their own will — 'I (have) decided to do…'. Attaches to the dictionary form (to do) or ない form (decide not to do). The た-form ことにした reports a decision already made; ことにしている expresses an established habit or rule one keeps to.
〜ことにする — how do you form it?
V-る／V-ない＋ことにする
〜ことにする — any usage notes?
Contrast with 〜ことになる, which expresses a decision/outcome reached by circumstances or others (not one's own will).