In short

이/가 marks the grammatical subject - the noun that does the action or carries the description. 은/는 marks the topic - the thing you have chosen to talk about. Each comes in two shapes, and the shape is decided purely by whether the noun ends in a 받침: 이 and 은 after a 받침, 가 and 는 after a vowel.

The rule

Step 1 - Choose the shape from the 받침, before anything else

Korean particles glue onto the end of a noun with no space. Both of today’s particles are pairs, and picking the wrong half is not a style slip - it is a spelling error that a Korean reader notices instantly.

• Noun ends in a consonant (받침) -> 이, 은. 학생 -> 학생이 (haksaengi), 학생은 (haksaengeun).
• Noun ends in a vowel -> 가, 는. 커피 -> 커피가 (keopiga), 커피는 (keopineun).

ㄹ counts as a 받침 like any other: 물 -> 물이 (muri), 서울 -> 서울은 (Seoureun).

Step 2 - 이/가 = the subject

Noun + 이/가 + Predicate. This is the noun the sentence is about grammatically: the one that acts, exists, or has the quality being described.

• 날씨가 좋아요. (Nalssiga joayo.) - The weather is nice.
• 동생이 학교에 가요. (Dongsaengi hakgyoe gayo.) - My younger sibling goes to school.

Question words in subject position take 이/가, never 은/는 - you cannot make an unknown thing your topic.

Step 3 - 은/는 = what you are talking about

Noun + 은/는 + [the rest of the sentence]. Read it as “as for X…”. It announces the thing under discussion and then says something about it - a self-introduction, a general truth, a heading for the sentence.

• 김치는 매워요. (Gimchineun maewoyo.) - Kimchi is spicy. (kimchi in general, as a subject of discussion)

은/는 is NOT “the subject marker”. English-language materials often mislabel it that way, and it is the single most damaging thing a beginner can believe about Korean. 은/는 says this is the theme; the theme very often happens to also be the subject, which is why the two look interchangeable at first. Sometimes it sits on something that is not the subject at all - see Step 6.

The full 은/는 vs 이/가 contrast is deferred to When 은/는, when 이/가. New information vs old, contrast, exclusive focus, what happens in subordinate clauses - none of that is your job right now. At A1, use the two defaults in Steps 4 and 5 and you will be right most of the time.

Step 4 - The self-introduction frame: 저는 …이에요

Memorise this whole frame as one chunk. It is the highest-frequency sentence you will ever build in Korean:

저는 + [noun] + 이에요/예요.

• 저는 학생이에요. (Jeoneun haksaengieyo.) - I am a student.
• 저는 의사예요. (Jeoneun uisayeyo.) - I am a doctor.

The copula is itself an allomorph pair (이에요 after a 받침, 예요 after a vowel - see Saying what something is: 이다 and 아니다), so one short sentence makes you choose correctly twice.

Step 5 - Five fusions you must learn by heart

Two pronouns and one question word do not simply add the particle - they fuse with it. The unfused forms are wrong, not merely unusual.

• 누구 + 가 -> 누가 (nuga) - who (as subject)
• 나 + 가 -> 내가 (naega) - I (casual)
• 저 + 가 -> 제가 (jega) - I (polite)
• 나 + 는 -> 난 (nan) - casual, shortened
• 저 + 는 -> 전 (jeon) - polite, shortened

With 은/는 the full forms 나는 and 저는 are perfectly normal writing; 난 and 전 are just faster spoken contractions. With 이/가 there is no choice at all: 누가, 내가, 제가 are the only forms.

Step 6 - Both particles in one sentence

Because the two do different jobs, one sentence can carry both - a topic first, then a subject inside the comment:

저는 + 이름이 + [name]예요.

• 저는 이름이 지수예요. (Jeoneun ireumi Jisuyeyo.) - My name is Jisu. Literally as for me, the name is Jisu.

Here 저는 is the topic (the person under discussion) and 이름이 is the grammatical subject (the thing that is Jisu). This is proof that 은/는 and 이/가 are not competing labels for one slot. You will meet this pattern properly at When 은/는, when 이/가; for now just recognise it.

Forms & patterns

The subject particle 이/가:

The topic particle 은/는:

ㄹ is a 받침 like any other here. 물 -> 물이 (muri), 서울 -> 서울은 (Seoureun) - no special case, because these particles only ask “consonant or vowel?”. ㄹ becomes special later, with endings that begin with 으: 서울 -> 서울로 (Seoullo), never *서울으로. Do not carry that exception back to 이/가 and 은/는.

The fused forms:

What each particle is doing:

Examples

Common mistakes

• 커피이 맛있어요. (Keopii masisseoyo.) -> 커피가 맛있어요. (Keopiga masisseoyo.) - 커피 ends in a vowel, so the subject particle is 가. 이 only ever follows a 받침.
• 선생님는 한국 사람이에요. (Seonsaengnimneun Hanguk saramieyo.) -> 선생님은 한국 사람이에요. (Seonsaengnimeun Hanguk saramieyo.) - 님 has the 받침 ㅁ, so the topic particle is 은. Check the last syllable of the actual word every time.
• 누구가 왔어요? (Nuguga wasseoyo?) -> 누가 왔어요? (Nuga wasseoyo?) - 누구 + 가 fuses to 누가. The unfused form does not exist.
• 저가 학생이에요. (Jeoga haksaengieyo.) -> 제가 학생이에요. (Jega haksaengieyo.) - 저 + 가 fuses to 제가, exactly as 나 + 가 fuses to 내가.
• 저는 학생이 있어요. (Jeoneun haksaengi isseoyo.) -> 저는 학생이에요. (Jeoneun haksaengieyo.) - 있어요 means there is / I have; saying what someone is needs the copula 이에요/예요.

Tips

• The sound-based mnemonic. A noun ending in a vowel takes the particle that starts with a consonant (가, 는); a noun ending in a 받침 takes the one that starts with a vowel (이, 은), so the 받침 can slide over into it: 물 + 이 is pronounced [무리] (muri). Korean is avoiding two vowels or two consonants crashing together - that is the whole system.
• Translate 은/는 as “as for”, never as “is”. As for me, student. Awkward English, but it keeps you from treating 은/는 as a subject marker or as part of the verb.
• Learn 저는 …이에요/예요 as one unbreakable chunk. It is your first complete sentence, and it drills both allomorph pairs at once. Say 저는 학생이에요 and 저는 의사예요 out loud until the choice is automatic.
• Do not agonise over 은/는 vs 이/가 yet. At A1 the defaults carry you: 은/는 for introducing yourself and for general statements, 이/가 for the doer and for answering who? or what?. The real contrast is a whole lesson of its own later on.