In order to express your curiosity as to "WHAT" is good, you can ask:
뭐가 좋아요? [mwo-GA jo-a-yo?]
Here the word 가 [ga] emphasize what the subject of the verb "to be / 좋아요" is.
If one says,
ABC 좋아요. [ABC jo-a-yo] = ABC is good.
And if you disagree and you think XYZ is good, not ABC. You can express your opinion that the subject of "being good" should be XYZ, not ABC, by saying:
ABC 좋아요? XYZ가 좋아요! [ABC jo-a-yo? XYZ-GA jo-a-yo]
So, with 이 and 가, you can add more flavor and more concrete meanings to your Korean sen-
tences when you want to emphasize WHO did something, WHICH ONE is good, and etc.
Is that everything?
As you can see from above, 은/는 and 이/가 both have different roles, but the roles of 은/는 as a 'contrast' factor is much stronger, because you can change the topic of a sentence with 은/는.
So, when you form complex sentences (i.e. I think the book that you bought is more interesting than the book I bought.), in general, 은/는 is not so commonly used all over the sentences. Often times 은/는/이/가 can be dropped, but when you need particles to clarify the meaning, 이/가 will be more commonly used.
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Let's Learn Korean [Compilation]Non-Fiction
Highest Rank: #1 Non-Fiction Annyeonghaseyo. Jeoneun Man Joo Yeon imnida. I'm here to help you to learn some korean phrases,grammars or words in an easy way here in this book. DISCLAIMER: This is just a compilations! Credits to the sources I got...