What it means
ことにする expresses a decision made by the speaker or another person. It follows the plain form of a verb: the dictionary form indicates deciding to do something, while the nai-form indicates deciding not to do something. It often describes a personal choice, a resolution, or a decision about a future action. The polite ending ことにします is used in polite speech and writing.
Usage note
Do not confuse ことにする with ことになる. ことにする emphasizes that someone makes the decision, while ことになる describes a decision or arrangement that has been determined, often without emphasizing who decided it.
How to use it
来月から毎朝走ることにします。
I have decided to run every morning starting next month.
甘い飲み物を飲まないことにします。
I have decided not to drink sweet drinks.
来月から毎朝走ることにする。
I have decided to run every morning starting next month.
甘い飲み物を飲まないことにする。
I have decided not to drink sweet drinks.
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.
今年の夏は、家族と北海道へ旅行することにしました。
This summer, I decided to travel to Hokkaido with my family.
来月から毎晩寝る前に本を読むことにします。
I have decided to read a book every night before going to bed starting next month.