Here is a simple example : 누가 갔니? Who went? 내가 갔어 It was I who went (and not Mary, Bob, or Jo). ***** 이번 주말에 무엇을 했니? What did you do this weekend? 나는 공원에 갔어 (As a matter of fact) I went to the park. ### Also, I realize that this is complicated by the fact that the words for "I" and "you" are similar, and we use a different word depending on the particle used as well as variants. I am not sure how I could make that easier.

Interestingly enough, there was exactly the same question in what the difference is in Japanese that I just answered today. The same principle is applied here. 는 is a general topic marker to express a general, neutral fact. 가 is a topic marker used when the topic was already introduced or the reference to the topic is clear. As such, these are called "particles" and there are many of them. In your example, 네가 친구랑 갔니? (here I ignore the issue of honorific or familiar modes of speaking) The implication is that you are being singled out. It's more like, "was it YOU who went with your friend" (in exclusion of other possible people). 너는 친구랑 갔니? This simply asks whether it is true that you went with a friend.

If the explanation is still unclear, we can examine a few more examples. I realize this is really weird because this sort of thing is absent in English.