A1
이/가 — the subject
How the Korean subject particle 이/가 differs from the topic particle 은/는, and the situations that require it.
이/가 marks the subject — the one doing or being. 이 after a consonant, 가 after a vowel.
비가 와요.
Biga wayo.
It’s raining.
친구가 왔어요.
Chinguga wasseoyo.
My friend came.
Against 은/는
Both can sit on the subject, and the choice changes the emphasis rather than the grammar. 은/는 sets a topic; 이/가 presents new information or picks one out from others.
누가 왔어요?
Nuga wasseoyo?
Who came?
누구는 왔어요?
누가 왔어요?
A question word is the new information by definition, so it takes 이/가 and never the topic marker. The answer keeps it too: 친구가 왔어요.
New things take 이/가, then become 은/는
Introduce something with 이/가; once it is established, it is the topic.
옛날에 할머니가 살았어요. 할머니는 아주 가난했어요.
Yennare halmeoniga sarasseoyo. Halmeonineun aju gananhaesseoyo.
Long ago there was an old woman. She was very poor.
The first sentence introduces her; the second talks about her. Swapping the two particles would sound wrong to a Korean ear in a way that is hard to explain and easy to hear.
Some verbs and adjectives require it
좋다, 싫다, 있다, 없다, 필요하다 take 이/가 on the thing liked, disliked, present or needed — where English would make it an object.
시간이 없어요.
Sigani eopseoyo.
I haven’t got time.
물이 필요해요.
Muri piryohaeyo.
I need water.
나 and 저 change shape
Before 가, 나 becomes 내 and 저 becomes 제: 내가, 제가. 누구 becomes 누: 누가.