This is the question every Korean learner asks, and the standard answer — 은/는 marks the topic, 이/가 marks the subject — is technically correct and practically useless. Both sentences below are grammatical, and neither is "the subject" in a way you can feel:
- 저는 학생이에요.
- 제가 학생이에요.
The difference is real, and Koreans hear it immediately. It just is not a difference about grammar roles. It is a difference about what is new and what is being contrasted.
First, the mechanical part
| After a consonant | After a vowel |
|---|---|
| 은 | 는 |
| 이 | 가 |
선생님은 · 저는 · 선생님이 · 제가
Note that 나 and 저 become 내가 and 제가 with 이/가. Irregular, and very common.
Situation 1: New information takes 이/가
When something enters the conversation for the first time, it takes 이/가. Once it is established, it switches to 은/는.
옛날에 한 남자가 살았어요. 그 남자는 아주 가난했어요. Once there was a man. The man was very poor.
This is exactly like English switching from a to the. First mention introduces; second mention refers back.
If you notice yourself using 은/는 for something you have not mentioned yet, that is usually the mistake.
Situation 2: Question words always take 이/가
This one is a hard rule and it is the fastest way to get the particle right.
누가 왔어요? — Who came? 지수가 왔어요. — Jisu came.
Not 지수는 왔어요. The question asks which person, so the answer supplies new information, so it takes 이/가.
Compare:
지수는 언제 왔어요? — When did Jisu come? 지수는 어제 왔어요. — Jisu came yesterday.
Here Jisu is already known and the new part is the time, so Jisu takes 은/는.
The test: whatever answers the question word takes 이/가. Everything already known takes 은/는.
Situation 3: 은/는 implies contrast
This is the meaning English speakers keep missing.
커피는 안 마셔요.
This does not just mean "I don't drink coffee." It means coffee, I don't drink — implying there is something else you do drink. A Korean listener will hear the implied "but tea, yes."
저는 괜찮아요. — I'm fine (whatever the others are).
This is why 은/는 can attach to objects, times, and places, not just subjects. It is not a subject marker at all. It is a spotlight that says "as for this one, specifically."
Situation 4: General truths take 은/는
고양이는 귀여워요. — Cats are cute. 한국 음식은 매워요. — Korean food is spicy.
Statements about a category as a whole use 은/는. Using 이/가 here would sound like you are pointing at one specific cat.
Both in one sentence
This combination confuses people and it is extremely common:
저는 커피가 좋아요. — I like coffee.
저 is the topic — what the sentence is about. 커피 takes 이/가 because 좋다 is an adjective meaning "is good," so literally this is as for me, coffee is good.
Same structure:
저는 한국어가 어려워요. — Korean is hard for me. 저는 시간이 없어요. — I don't have time.
Verbs and adjectives like 좋다, 싫다, 있다, 없다, 어렵다 take 이/가 on the thing being described, not 을/를.
When you can drop them
In casual speech, particles disappear constantly.
커피 마셔? — Drinking coffee? 나 갈게. — I'll go.
This is normal and natural. But dropping a particle removes the information it carried, so it only works when context makes everything obvious. Early on, keep them.
The three mistakes to watch for
Using 은/는 in answers to question words. 누가 왔어요? → 지수가 왔어요. Always 이/가.
Using 은/는 for something brand new. Introduce with 이/가 first.
Not hearing the contrast. If you say 저는 좋아요 when nobody asked about anyone else, it can sound like you are distancing yourself from the group.
How this becomes automatic
No rule list makes this automatic. What does is hearing the same structures in context often enough that the wrong one starts sounding wrong.
Dialogue helps more than isolated sentences here, because the topic/new-information distinction only exists across a conversation. In a single sentence there is nothing to contrast with.
K-PATTO episodes are short conversations where you see the same particles doing this work over and over, with audio. The first ten are free at K-PATTO.