i'm not a native speaker but it's just from what i learned -이/가 -이/가 marks the subject in the sentence. -이 comes after a noun that ends in a consonant, and 가 comes after a noun that ends in a vowel. Examples) 1. 마크가 운동을 한다. Mark does exercise. 마크(Mark) is the subject, and 운동(exercise) is the object. 2. 지갑이 없어졌다. (My) wallet is gone. 지갑(wallet) is the subject. 3. 수잔이 집에 있다. Susan is at home. 수잔(Susan) is the subject. What's the difference between -이/가 and -은/는? To talk about new topics, use -이/가 at first, and then use -은/는. Examples) 1. 나비가 하늘을 날아다닙니다. 나비는 꽃에 다가갔습니다. The butterfly flies in the sky. It approached to flowers. 2. 저기 패트릭이 온다. There, Patrick comes. -이/가 is for general statements, while 은/는 is for comparing statements. Examples) 1. 하늘이 파랗다. The sky is blue. 2. 강아지가 짖는다. The puppy barks. 3. 하늘은 파랗다. (하지만 비가 온다.) The sky is blue. (But it rains) 4. 강아지는 짖는다. (하지만 고양이는 안짖는다.) The puppy barks. But the cat doesn't (bark). -이/가's focus is the noun in front of the sentence, while -은/는 focuses the noun in the end. Examples) 말리가 서점에 갔어요. Marley went to a book store. (The sentence is focused on Marley) 말리는 서점에 갔어요. Marley went to a book store. (The sentence is focused on a book store) For endings -겠습니다, -으ㄹ까요, use -이/가. Examples) 1. 제가 도와드리겠습니다. I will help (you). 2. 제가 먹을까요? Shall I eat (this)?