Grammar · N4
〜ことにする vs 〜ことになる
Distinguish a decision you make from a decision or arrangement that comes about.
In short
〜ことにする presents the subject as deciding or choosing: 毎日歩くことにしました. 〜ことになる presents a decision, rule or arrangement as established by circumstances or another authority: 来月転勤することになりました.
How it works
The difference is viewpoint: personal agency versus an outcome presented as externally determined or settled.
Useful contrast
In real situations the same event can be described either way depending on what the speaker wants to foreground.
Examples
毎日読むことにしました。
I decided to read every day.
来月大阪へ行くことになりました。
It has been decided that I will go to Osaka next month.
Common pitfall
Do not translate ことになる as a literal passive every time. Natural English may be “it has been decided”, “I am going to”, or “the arrangement is”.