We make many decisions throughout our lives. We resolve to do things happen. This we need to be able to express in Korean and that is not that difficult. One quick and easy form to use to express that is -기로 했다.
The form -기로 했다 is a simple expression you use to indicate a promise you made with someone else or with yourself. In short, it indicates you promise to do something or make something happen. The difference with a form like -(으)ㄹ게요 is that you can use these forms not only in casual situations, making it a bit more useful to know. Do notice that this form is typically used in the past tense because it expresses the promise that has been made.
It is possible to use the form in the present tense, -기로 하다, but it is only used so when you are currently deciding to do things together. So it is only used when in the middle of making plans together.
가: 주말에 뭐 할까요? (What are we going to do during the weekend?)
나: 쇼핑하기로 해요. (Let’s go shopping.)
가: 새해에 무슨 계획이 있나요?
나: 새해에는 독일어를 배우기로 했어요.
A: What plans do you have for the new year?
B: I decided to learn German.
How To Use
This form -기로 했다 is very simple to use. To use it with a verb, it can only be used with verbs, you take the verb’s stem and you attach the ending to it. This is one of the ‘no rule’ verb endings in Korean grammar. As a bonus, you needn’t worry over the Korean irregular verbs since they all play nice with this ending. However, don’t forget you still need to conjugate the 했다 part of the form. Otherwise, your sentence is incomplete.