은/는 and 이/가: topic and subject

A particle is a small ending joined to a word to show its role in the sentence. Many particles have two forms. The choice depends on the last letter of the word: a word ending in a consonant (a batchim) takes one form, a word ending in a vowel takes the other.

The topic particle is -은 after a consonant and -는 after a vowel. The subject particle is -이 after a consonant and -가 after a vowel. After a consonant, the batchim links into the particle: 책은 is said chae-geun, 약국이 ya-gu-gi.

은 / 는 marks the topic: what the sentence is about. Think of it as “as for …”: 저는 서연이에요 (jeoneun Seoyeonieyo) is literally “as for me, (I'm) Seoyeon”. It's the natural choice when you introduce yourself or talk about something already known.

It also marks contrast: this one, as opposed to others. 커피는 좋아해요 (keopineun joahaeyo) means “coffee, I like”, which suggests there is something else you don't like. Time words often take it for the same reason: 오늘은 바빠요 (oneureun bappayo), today I'm busy (but other days I'm not).

이 / 가 marks the subject when it is new information or the important part of the sentence. English often shows this with stress: “The bus is coming.”

Question words can't be a topic, because they aren't known yet, so they take 이 / 가, and so does the answer: 뭐가 맛있어요? (mwoga masisseoyo?, what's good here?) → 비빔밥이 맛있어요 (bibimbabi masisseoyo, the bibimbap is good). 이 / 가 is also used for something you have just noticed: 비가 와요 (biga wayo), it's raining.

A few words change before 가: 저 → 제가 (jega, I), 나 → 내가 (naega, I, casual), 누구 → 누가 (nuga, who).

Some verbs describe a state rather than an action, and the thing in that state takes 이 / 가. With 있어요 (isseoyo, there is) and 없어요 (eopseoyo, there isn't), the thing that exists takes 이 / 가: 근처에 약국이 있어요? (geuncheoe yakgugi isseoyo?, is there a pharmacy nearby?).

The same goes for feelings and conditions. With 좋아요 (joayo, is good, is liked) the thing you like takes 이 / 가: 저는 여름이 좋아요 (jeoneun yeoreumi joayo), I like summer. With 아파요 (apayo, hurts) the body part does: 머리가 아파요 (meoriga apayo), my head hurts. Notice the pattern “person 은 / 는, thing 이 / 가”.

아니에요 (anieyo, is not) also takes 이 / 가: 학생이 아니에요 (haksaengi anieyo), I'm not a student.

Both mean “I”. 저는 (jeoneun) sets you as the topic: use it to introduce yourself or to contrast yourself with others, as in 저는요? (jeoneunyo?, what about me?). 제가 (jega) picks you out as the one who does something, often in answer to “who?”.

So if someone asks 누가 민준 씨예요? (nuga Minjun ssiyeyo?, which of you is Minjun?), the answer is 제가 민준이에요 (jega Minjunieyo), I'm Minjun. And when you offer to do something, you say 제가 할게요 (jega halgeyo), I'll do it.