We use the below structure to show that we have decided on one thing out of multiple choices. The most common example would be saying, “I’ll have the pizza” at a restaurant.
[NOUN] にする
私はピザにします。
I’ll take the pizza.
何にする？
What will you have?
Deciding on an Action (ことにする)
The above is the most basic structure. More commonly, you will see it used with nominalised verbs (Dictionary Form + こと) to show that you have decided to perform an action.
大学で専攻として歴史を選ぶことにしました。
I decided to choose history as my subject at university.
新しいテレビを買うことにしました。
I decided to buy a new television.
JLPT Trap (Negative Decisions): If you want to say “I decided not to do something,” you must make the verb negative, not the deciding action. It is 〜ないことにする (I decided not to do X), never 〜することにしない.
明日は会社に行かないことにした。
I decided not to go to work tomorrow.
ことにする vs. 決める
We could, of course, also use the verb “to decide” (決める – きめる).
新しいテレビを買うことに決めました。
I decided to buy a new television.
新しいテレビを買うことを決めました。
I decided to buy a new television.
The meaning is almost identical, with the following two points to note:
ことにする is more conversational and colloquial.
The ultimate meanings of the final verbs “to do” and “to decide” are still present in the sentence, and so the two are not always interchangeable.
For example, there is a special, highly tested usage of ことにする. When attached to the past tense (た-form) or negative past tense of a verb, it means “to treat something as if it happened” (even when it didn’t) or “to pretend.” You cannot use 決める here.
今の話は聞かなかったことにしてください。
Please pretend you didn’t hear what I just said (Let’s treat it as if you didn’t hear it).
今の話は聞かなかったことに決めてください。(✘)
Please decide to not have heard what I just said.
Making it a Rule: ことにしている
A major JLPT grammar point builds directly on this structure. If you change the verb to the continuous している form, it means you have decided on an action and continue to enforce it as a personal habit or rule (“I make it a rule to…”).
毎朝ジョギングをすることにしています。
I make it a rule to go jogging every morning.
夜１０時以降はスマホを見ないことにしている。
I make it a point not to look at my phone after 10 PM.