English tells you who did what by word order. Korean tells you with little tags glued to each noun. That is why Korean word order can shuffle and the sentence still makes sense.
이 / 가the subject — who does it · 친구가 / 눈이
은 / 는the topic — “as for…” · 저는 / 학생은
을 / 를the object — what it happens to · 영화를 / 책을
The pattern repeats everywhere: the form with the extra vowel goes after a consonant. 이, 은, 을 follow consonants. 가, 는, 를 follow vowels. It is purely about keeping words easy to say.
이/가 vs 은/는 — the one everyone struggles with
Rough rule that gets you far: 은/는 introduces or contrasts (“as for me…”), 이/가 points at who specifically did something. Native speakers feel this rather than think it, and so will you, eventually.
저는 학생이에요 · jeo-neun hak-saeng-i-e-yoAs for me, I am a student. Introducing yourself.
제가 했어요 · je-ga hae-sseo-yo*I* did it. Answering “who did this?”
연습 · Practice
이 레슨에는 풀어 보는 문제가 5개 있습니다. 앱에서 하나씩 맞히며 넘어갑니다. 5 exercises come with this lesson — open it to work through them.