You use this to say what you have decided to do. It shows that you made the choice yourself.
Verb [dictionary form] + ことにするPrimary
Verb [ない-form] + ことにする
You use this to say what you have decided to do. It shows that you made the choice yourself.
明日から健康のために、毎朝ジョギングすることにする。
For my health, I've decided to start jogging every morning from tomorrow.
環境のことを考えて、もうレジ袋はもらわないことにします。
Thinking about the environment, I've decided not to take plastic shopping bags anymore.
悩んだ末、会社を辞めることにしたんだ。
After a lot of thought, I decided to quit my job.
留学することにしたので、これからその準備を始めます。
Since I've decided to study abroad, I will start preparing for it now.
色々見たけど、やっぱりこの青いドレスを買うことにするわ。
I've looked at a bunch, but I've decided to buy this blue dress after all.
This is different from "〜ことにしている". That means you do something regularly, like a habit. This is also different from "〜ようにする". That means you try to do something. "〜ことにする" is a stronger choice.
Watch out: Don't mix up 〜ことにする and 〜ことになる. You use 〜ことにする when you decide something yourself. For example, "I decided to eat sushi." You use 〜ことになる when someone else decides for you. Or when a rule makes it happen. For example, "It was decided that I will move." You did not choose to move yourself.
Don't use this when talking about someone else's decision. For example, don't say "He decided to go." This phrase is for your own choices. It shows you made a choice for yourself.
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