Learn Hardcore Korean: eonjenga chingureul dasi mannal geopnida.

Breakdown of eonjenga chingureul dasi mannal geopnida.

What does 언젠가 mean here?

언젠가 means someday, one day, or at some point in the future.

In this sentence, it gives the whole statement a future, indefinite time feeling:

It does not mean a specific date. It just means the speaker believes the meeting will happen at some unknown time.

Why is there no subject like I or we in the sentence?

Korean often leaves out the subject when it is understood from context.

So 언젠가 친구를 다시 만날 겁니다 can mean:

In real conversation, the listener usually knows who the subject is from the situation. Korean does this much more often than English.

Why does 친구 have 를 after it?

를 is the object particle. It marks 친구 as the thing receiving the action of the verb.

Since 만나다 means to meet, the person being met is the object:

So in this sentence, 친구를 means (one’s) friend as the object of meet.