When to use 은/는 or 이/가 after subject

Question

I keep missing questions bc I don't know when to use 은/는 or 이/가 after the subject. Ex. 엄마가 냉면을 좋아합니다. 저도 컴퓨터가 있습니다.

이 컴퓨터는 얼마 입니까? 할아버지는 선생님 입니다. Can you help?

Answer 1

이/가 is used to "mark" the subject (preceding 가) that you're attempting to direct a little more attention to. Often omitted.

은/는 is a "stronger" marker, I would say. It can be used to direct your listener's/reader's attention to what the overarching topic is that you're going to talk, perhaps at some length, about. It is also used to contrast.

이 컴퓨터(가) 얼마입니까? -> This computer... how much is it?

이 컴퓨터는 얼마입니까? -> How much is THIS computer (as opposed or compared to the others)?

할아버지는 선생님입니다. Someone would likely say this, with 는, after they have just finished telling you the occupation of someone else, such as 할머니.

요리는... If one starts a sentence like this, they are probably about to speak at length, or in some comprehensive way, about the act of cooking, and they want you to understand that it's the main topic. And/Or they may be contrasting it with some other act they were just talking about, such as 가르치기.

Answer 2

The subject marker 이/가 is for new information or focusing on subject.

The topic marker 은/는 is an auxiliary particle 보조사 and can replace 주격조사 이, 가. It is for old information, contrast/comparison or focusing on description.

저기 집이 있습니다. 그 집은 산 위에 있습니다.

누가 파티에 갔어? -> 지민이가 파티에 갔어 (focus on the subject)

지민이 뭐 해? -> 지민이는 파티에 갔어. (focus on the description)

Answer 3

• If subject don't have final consonant you can use that sub + (-는) : Cat[고양이+(-는)] likes fish sun + (-가) : Cat[고양이+(-가)] eats fish

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• If subject have final cosonant you can use that sub + (-은) :Reindeer[순록+(-은)] are part of the deer family. sub + (-이) :Reindeer[순록+(-이)] is moving.

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• 마 of 엄마 don't have final consonant you can use both (-는)(-가) as contents by meaning
• 터 of 컴퓨터 don't have final consonant you can use both (-는)(-가) as contents by meaning
• 지 of 할아버지 don't have final consonant you can use both (-는)(-가) as contents by meaning

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• however 님 of 선생님 have final consonant so you can use both (-은)(-이)