What is the difference between 은/는 and 이/가 in Korean?

The forms

Two subject forms change shape: 저 + 가 → 제가 (I), 나 + 가 → 내가 (I, casual), and 누구 + 가 → 누가 (who).

은/는: the topic

Use 은/는 to say what you are talking about, especially when it is already known:

• 저는 간호사예요.

  I’m a nurse.

  introducing yourself: the topic is “me”

• 이 식당은 정말 맛있어요.

  This restaurant is really good.

  the restaurant is the topic

And to contrast two things:

• 커피는 좋아해요. 차는 안 좋아해요.

  I like coffee. Tea, I don’t like.

  contrast

• 오늘은 바빠요.

  Today I’m busy (but other days, maybe not).

  implied contrast

이/가: the subject

Use 이/가 when the subject is new, identified, or specified:

• 누가 왔어요? — 민수가 왔어요.

  Who came? — Minsu came.

  answering “who”

• 비가 와요.

  It’s raining.

  describing what is happening now

• 저기 버스가 와요.

  There comes the bus.

  something newly noticed

Where 이/가 is required

Some structures use 이/가 by default:

• Question words as subjects always take 이/가: 누가, 뭐가, 어디가, 언제가.
• 있다 / 없다 (to have / not have, to exist): 시간이 있어요 (I have time), 돈이 없어요 (I don’t have money).
• 아니다 (to not be): 저는 학생이 아니에요 (I’m not a student).
• 되다 (to become): 의사가 되고 싶어요 (I want to become a doctor).

Side by side

• 저는 민수예요.

  I’m Minsu.

  a normal self-introduction

• 제가 민수예요.

  I’m (the one who is) Minsu.

  answering “Which of you is Minsu?”

• 이 사람은 선생님이에요.

  This person is a teacher.

  describing the topic

• 이 사람이 선생님이에요.

  This is the teacher.

  identifying who the teacher is

Common mistakes

• ✗Not: 누구는 왔어요?

  ✓Better: 누가 왔어요?

  question-word subjects take 이/가

• ✗Not: 저가 할게요.

  ✓Better: 제가 할게요.

  저 + 가 becomes 제가

• ✗Not: 시간은 없어요. (as a neutral statement)

  ✓Better: 시간이 없어요.

  시간은 없어요 is possible, but it implies contrast (“time, I don’t have”)

• ✗Not: 책는

  ✓Better: 책은

  after a consonant: 은

Both in a short conversation

Seeing both particles in one exchange makes the difference clearer:

• A: 누가 커피를 샀어요?

  A: Who bought the coffee?

  question word → 이/가

• B: 제가 샀어요.

  B: I did.

  answering “who” → 제가

• A: 고마워요. 저는 차를 좋아해요.

  A: Thanks. I (for my part) like tea.

  저는: topic, with a hint of contrast

• B: 아, 차는 없어요. 커피가 있어요.

  B: Oh, there’s no tea. There’s coffee.

  차는: contrasted topic; 커피가: subject of 있어요

In the last line, 차는 sets tea apart as the topic (“as for tea…”), while 커피가 is the plain subject of 있어요 (there is). Swapping them would change the emphasis, not the facts — which is exactly how the two particles differ.

A practical approach

1. Use 은/는 to introduce yourself and the main topic of a conversation.
2. Use 이/가 with question words, with 있다/없다, and when answering “who/what”.
3. When you contrast two things, use 은/는 on both.
4. Do not worry about perfection: native speakers understand you either way, and the nuance becomes clear with lots of listening.