Grammar JLPT Level N3 - Lesson 9

Structure of 事(こと)にする:

Verb［る］+ ことにする
Verb［ない］+ ことにする

Details about 事(こと)にする:

In a similar way to how にする means to 'decide on' something, ことにする means to decide on a situation, or course of action. This grammar point is a combination of 事(こと) 'thing' in its role as a nominalizer (a word that creates a noun-phrase), and にする 'to make it (A)'. Literally this grammar structure can be translated as 'to do (A)', or 'to make it (A)'.
ことにする may be used with verb-phrases that are either in the affirmative, or negative (using ない) forms. As こと itself is a noun, the verb (or auxiliary verb) that comes before it will need to be in its attributive form.

今日(きょう)はワクチンを打(う)ったから、明日(あした)は仕事(しごと)を休(やす)むことにする。
Today I got vaccinated, so I will go with taking the day off tomorrow.
タマキさんと箱根(はこね)に行(い)くことにする。
I have decided to go to Hakone with Tamaki-san.
今日(きょう)は雨(あめ)が降(ふ)っているから、明日(あした)行(い)くことにしない？
It is raining today, don't you want to make it that we go tomorrow?
勝手(かって)に僕(ぼく)が行(い)くという事(こと)にしないでください。
Please don't decide that I will go without asking me. (Please don't assume that I want to go)

にする is a strong expression, which tends to make the speaker sound like they are fairly resolute in their decision.

事(こと)にする

Usage Notes for 事(こと)にする:

The する in にする itself may also be seen using ない, to mean that someone has decided 'against' doing something. Due to this, careful attention will need to be paid to the overall structure of the sentence.