verb + 기로 하다. decide to do

(tailieutienghan.com) (hompi.sogang.ac.kr)

The pattern '-기로 하다.' : decide to do

The pattern '-기로 하다' is used to indicate one's decision, making a choice between alternatives, or arriving at a solution to an uncertainty or dispute. So, this pattern is usually used in the past tense.

담배를 끊기로 했어요. I decided to quit smoking.
술을 마시지 않기로 했어요. I decided not to drink.
이번 주말에 여행을 가기로 했어요. I decided to go on a trip to Pusan.

In this form, the verb `-하다' can be replaced by the verb `약속하다, 결정하다, 결심하다, 작정하다' etc. See the following examples :

담배를 끊기로 결정했어요. I decided to quit smoking.
담배를 끊기로 약속했어요. I promised to quit smoking.
담배를 끊기로 결심했어요. I made up my mind to quit smoking.

There are two different ways of expressing the negative. In the first, when the pattern is preceded by a negative '-지 않기로 하다', it means 'to decide not to do'. In the second, when the negative is expressed in the verb '-기로 하다` itself, like '-기로 하지 않다', it means 'do not decide to do'.

먹지 않기로 했어요. I decided not to eat.
먹기로 하지 않았어요. I haven't decided to eat.