In addition to the topic of 은/는 indicating emphasis and nuance, 이 / 가 has the feeling of “none other than” “nothing but” and also, when used inside a complex sentence, the role of marking the subject without emphasizing it too much. For example, if someone says '좋아요', it means 'it's good' but if you're not sure exactly what's good then you can ask '뭐가 좋아요?' ('what's good?'). If I say 커피 좋아요 (=Coffee is good) and you disagree because tea is better, then you can say "커피 좋아요? 차가 좋아요!" (=Coffee's good? Tea's good!). 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.