-기로 하다

-기로 하다 means that someone has decided, or agreed with someone else, to do something.

목적·의도Purpose, plans & wishes

Han: The past marker -았/었- cannot go before -기로; the tense of the decision is shown only on 하다 (했어요).

Examples

다음 달에 가족들과 제주도로 여행을 가기로 했어요.My family and I decided to take a trip to Jeju Island next month.

건강을 위해서 밤에는 라면을 먹지 않기로 했어요.For my health, I decided not to eat ramyeon at night.

주말에 친구하고 영화를 보기로 했어요.My friend and I agreed to see a movie on the weekend.

올해부터 담배를 끊기로 했어요.I decided to quit smoking starting this year.

V+기로 하다

가다 → 가기로 했어요

Tense goes on 하다, not on the verb before -기로: say 가기로 했어요, not 갔기로 했어요.
It is usually used in the past form (-기로 했어요) to report a decision already made; -기로 해요 or -기로 하자 is used to suggest deciding something together.
To decide NOT to do something, use -지 않기로 하다 (or 안 + V-기로 하다).
With a partner (친구하고, 친구와) it expresses a plan agreed on together, not just a personal decision.

Wrong: ~~내일 친구하고 만났기로 했어요.~~

Right: 내일 친구하고 만나기로 했어요.

The past marker -았/었- cannot go before -기로; the tense of the decision is shown only on 하다 (했어요).

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