事(こと)にするTo decide on, Have decided to

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.

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

Caution

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.