LEARN + JAPANESE / Note 113 JLPT N4

It Was Decided, I Decided (ことになる・ことにする)

ことになる attaches after a verb’s plain form and reports that something has been decided, usually by circumstances or by someone else, not by your own personal will. ことにする reports the opposite: a decision you made yourself, on purpose.

In Real Life
ことになりました almost always implies the decision came from outside the speaker, a company transfer, a school assignment, a circumstance that worked out this way, rather than a personal choice freely made.
In Real Life
ことにしました reports a one-time decision, “I decided to.” ことにしています, built with ている, reports an ongoing personal rule or habit you’ve committed to, “I make it a policy to,” a subtle but useful shift in nuance.
In Real Life
ことになっている, with ている on ことになる, describes a standing rule or schedule that’s already in place, often a workplace policy or an arrangement everyone’s expected to follow, rather than a single decision made once.

Vocabulary

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