Learn Hardcore Korean: chinguga jibe wasseoyo.

가 is the subject‐marking particle in Korean. You use 가 (or 이 after a consonant) when you’re simply indicating who or what is performing the action, especially when it’s new information. 은/는 are topic‐marking particles used to set up or contrast topics you’ve already introduced. Here, the focus is on “a friend” coming, so 친구가 is most natural.

What does 집에 mean? Why is the particle 에 used rather than 에서?

집 means “house/home.” The particle 에 after 집 indicates direction or destination (“to the house”). If you said 집에서, you’d be marking the location of an action (“at the house”) – for example, 집에서 쉬었어요 (“I rested at home”). Here, you’re talking about coming to the house, so you use 에.

What does 왔어요 mean, and how is it formed?

왔어요 is the polite informal past tense of the verb 오다 (“to come”). The irregularity is that the stem 오- changes to 왔- in the past, and you add the polite ending -어요, giving you 왔어요 (“(someone) came”).

Could I say 집에 친구가 왔어요 instead of 친구가 집에 왔어요? Does word order matter?

Korean word order is fairly flexible because the particles show each word’s role. The most neutral order is Subject → Location → Verb (친구가 집에 왔어요), but you can also say 집에 친구가 왔어요 without changing the basic meaning. Slight shifts in order can add emphasis (e.g., fronting 집에 emphasizes “to the house”).

How do I know 집에 왔어요 means “came to my (or the speaker’s) house” rather than “came to their own house”?

Because the verb 오다 means “to come” toward the speaker’s location. If you wanted to say “my friend went to their own house,” you’d use 가다 (“to go”): 친구가 집에 갔어요. Using 오다 always implies moving to where the speaker (or listener) is or will be.

Why is there no word for “he” or “she” in 친구가 집에 왔어요?

In Korean, it’s common to omit pronouns when the subject is clear from context. Here, 친구가 already tells you who did the action, so adding 그 (“he/she”) would be redundant (그 친구가 집에 왔어요 is possible but longer).

Can I change the politeness level of 왔어요? How?

Yes.

Formal polite: 왔습니다 (often in announcements or formal speeches).
Casual/informal: 왔어 (with close friends or younger people).
Very informal/plain: 왔니? (almost childlike or playful).
Choose based on your relationship and setting.