Decisions: ことにする

JLPT N4. Used in both writing and conversation.

We use the below structure to show that we have decided one thing out of multiple choices. The most common example would be say, “I’ll have the pizza” at a restaurant.

[NOUN] にする

私わたしはピザにします。

I’ll take the pizza.

何なににする？

What will you have?

This is the most basic structure. More commonly you will see it used with nominalised verbs to show that you have decided to do something.

大学だいがくで専攻せんこうとして歴史れきしを選えらぶことにしました。

I decided to choose history as my subject at university.

新あたらしいテレビを買かうことにしました。

I decided to buy a new television.

We could, of course, also use the verb “to decide”.

新あたらしいテレビを買かうことに決きめました。

I decided to buy a new television.

新あたらしいテレビを買かうことを決きめました。

I decided to buy a new television.

The meaning is almost identical with the following two points to note:

1. ことにする is more colloquial.
2. There 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. Look at the following two example sentences.

驚おどろいているふりにしてください。

Please do such that you pretend you are surprised.

驚おどろいているふりに決めてください。(✗)

Please decide to pretend that you are surprised.