이/가 marks the grammatical subject—the person or thing performing an action or being described. It identifies "who/what" specifically.
New information — Answering "who?" or "what?" questions
Existence/location — With 있다/없다, 있다/계시다
Descriptions — What something is like (adjectives)
Emphasis — Stressing the subject specifically
Use 이 after a consonant (시간이), 가 after a vowel (제가).
See examples below for varied real-world usage.
이/가 for new info, 은/는 for known — "누가 왔어요?" (who came?) → "민수가 왔어요" (Minsu came)
With 있다/없다 — Always use 이/가: "돈이 있어요" not "돈은 있어요" (unless contrasting)
저 vs 제 — 저 + 가 contracts to 제가, 나 + 가 contracts to 내가
Subordinate clauses — Subject in relative clauses typically uses 이/가, not 은/는
The weather (subject) is nice.
What (subject) is the problem?
I have many friends.
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