Decision

ことになる

Used for a decision that is made from an external factor. Means “it has been decided that”. Used with verbs.

Example

病気びょうきだから少し仕事しごとを休むことになった。 I had to take some time off work because I was sick.

ことにする

Used for a decision that is made by oneself. Means “to decide on”. Used with verbs.

Example

吸すうを止めることにしました。 I decided to stop smoking.

にする

See To decide.