What it means
ことにする expresses a decision made by the subject: deciding to do something, choosing a course of action, or setting a personal rule. It follows a verb in the dictionary form for an affirmative decision and a verb in the nai-form for a negative decision. The form focuses on the subject’s deliberate choice, while ことになる describes a decision or outcome determined by circumstances or another authority.
Usage note
Do not confuse ことにする with ことになる. Use ことにする when the subject makes the decision personally; use ことになる when the decision is settled by an outside situation, arrangement, or authority.
How to use it
来月から毎朝、駅まで歩くことにします。
I have decided to walk to the station every morning starting next month.
今週は外食しないことにします。
I have decided not to eat out this week.
来月から毎朝、駅まで歩くことにする。
I have decided to walk to the station every morning starting next month.
今週は外食しないことにする。
I have decided not to eat out this week.
Example sentences
健康のために、毎朝駅まで歩くことにしました。
For my health, I decided to walk to the station every morning.
来月から、仕事の予定を前日の夜に確認することにします。
I have decided to check my work schedule the night before starting next month.
今年は無駄な買い物をしないことにしました。
I decided not to make unnecessary purchases this year.