ことにする

The grammar structure ことにする expresses a personal decision to do (or not do) something. It emphasizes that the speaker has actively made a choice, as opposed to something being decided by circumstances or others.

Deciding to do something

Use the plain form of a verb before ことにする to express deciding to do something. The past form ことにした indicates a decision that has already been made.

Deciding not to do something

Use the negative plain form (ない-form) to express deciding not to do something.

Habitual decisions: ことにしている

When combined with 〜～ている, the form ことにしている means "I make it a rule to…" or "I've decided to (and continue doing so)". It describes a personal policy or ongoing practice.

ことにする vs. ことになる

While ことにする emphasizes a personal, active decision, ことになる indicates that something has been decided by external circumstances or by a group (not by the speaker alone).