은/는 vs 이/가

Beginner · TOPIK I

은/는 marks the topic — what the sentence is about, often known information or something being contrasted — while 이/가 marks the grammatical subject and puts the focus on new information, i.e. exactly who or what.

은/는A postpositional particle used to indicate that a certain subject contrasts with something else.이/가A postpositional particle referring to a subject under a certain state or situation, or the agent of an action.

Rules of thumb

Self-introductions, general truths and definitions take 은/는 (저는 학생이에요).
Question words like 누가, 뭐가 and their answers take 이/가, because they are the new, focused information.
Use 은/는 to contrast two things: 'A is …, but B is …'.
Inside subordinate and relative clauses (제가 만든 케이크 'the cake I made'), the subject normally takes 이/가, not 은/는.
With 있다/없다, 되다, 아니다, and when describing something you just noticed (비가 와요 'It's raining'), 이/가 is the default.

Example sentences

은/는
I'm Vietnamese.
이/가
Who came? — Minsu did.
은/는
I like apples, but not pears.
이/가
This is the cake I made.
이/가
Oh, it's raining outside.

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