Learn Hardcore Korean: ibeon jumare chinguwa bakmulgwane gagiro haesseoyo. - I decided to go to the museum with a friend this weekend.

-(으)기로 하다 is a grammar pattern that expresses a decision or a plan. When you put it in the past tense as 기로 했어요, it means “decided to (do something)” or “arranged to (do something).” So 가기로 했어요 literally means “(I/We) decided to go” or “(I/We) have plans to go.”

What is the function of 에 in 이번 주말에 and 박물관에?

The particle -에 serves two different roles here:

With time nouns like 이번 주말에, it marks “at/on” that point in time (“this weekend”).
With location nouns when paired with movement verbs like 가다, it marks the destination (“to the museum”).

Why is 친구와 used instead of 친구랑?

Both -와 and -랑 mean “with.”

-와 is slightly more formal or written.
-랑 is more colloquial and common in speech.
You can swap them without changing the meaning: 친구와 가기로 했어요 ≃ 친구랑 가기로 했어요.

Why is the subject of the sentence omitted?

Korean often drops the subject when it’s obvious from context. Here, the speaker (“I” or “we”) is clear, so you don’t need to say 저는 or 우리는.

Why isn’t there an object marker -을/를 on 박물관?

The verb 가다 (to go) is intransitive; it doesn’t take a direct object. Instead, the destination is marked by -에. That’s why you say 박물관에 가다, not 박물관을 가다.

Can you change the word order in this sentence?

Yes. Korean word order is flexible as long as:

The verb stays at the end.
Particles stay attached to their words.
For example:
- 친구와 이번 주말에 박물관에 가기로 했어요
- 이번 주말에 박물관에 친구와 가기로 했어요
  The original order (time – companion – place – verb) is simply the most natural.

What’s the difference between 가기로 했어요 and 갈 거예요?

가기로 했어요 emphasizes that a decision or plan was already made in the past (“we decided to go”).
갈 거예요 is a neutral future tense (“we will go”), expressing intention or prediction without highlighting when the decision was made.