Ask any Korean learner what trips them up most and you'll hear the same answer: 은/는 vs 이/가.
Both can mark the subject of a sentence. Both translate to nothing in English. And yet Koreans feel the difference immediately. And you will too, once you understand what each one actually does.
What they are
은/는 = topic marker (은 after consonant, 는 after vowel) 이/가 = subject marker (이 after consonant, 가 after vowel)
The core difference
은/는 says: "As for X, ..."
It marks what the sentence is about: the topic you're introducing or contrasting.
이/가 says: "X specifically, ..."
It marks the new information: the subject being introduced for the first time, or emphasized.
Example 1: Introducing vs continuing
저는 학생이에요.: As for me, I'm a student. (Introducing yourself; using 는 because "I" is the topic)
누가 학생이에요? 제가 학생이에요. Who is a student? I am a student. (Answering "who?". 가 marks the new subject being identified)
The rule: when answering a "who/what" question, use 이/가. When talking about a topic you've established, use 은/는.
Example 2: Contrast
저는 커피를 좋아해요, 근데 차는 싫어요: I like coffee, but as for tea, I don't like it.
는 on 차 signals contrast: "tea, on the other hand..."
This is the clearest use of 은/는: when you're contrasting two things, use the topic marker on both.
Example 3: Known vs new information
호랑이가 왔어요. A tiger came! (new, alarming information) 호랑이는 위험해요. Tigers are dangerous. (general known fact)
이/가 introduces the tiger as surprising new information. 은/는 treats tigers as an established topic being described.
The shortcut
When stuck, ask yourself:
- Am I contrasting this with something else? → 은/는
- Am I answering "who?" or "what?" → 이/가
- Am I making a general statement about a topic? → 은/는
- Am I pointing out something new or unexpected? → 이/가
Common mistakes
"저가 했어요" ✗. 저 + 가 doesn't exist. The humble first-person pronoun 저 becomes 제 before 가: 제가 했어요 ✓
"나는 했어요" vs "내가 했어요": both correct, different nuance. 나는 = "as for me, I did it" / 내가 = "I'm the one who did it"
The honest truth
Even advanced learners make mistakes with these. Native speakers sometimes pause. The difference is often subtle and context-dependent. The best way to internalize it is massive input: listening and reading until it feels wrong, not just until you can explain it. A structured Korean course drills sentence-level pattern recognition, which is exactly the kind of practice that makes this click.