The usage of 은/는 and 이/가

Group A 이/가 and group B 은/는 is used slightly differently. It goes by context. Of course they will be grammatically right if you mix them but it would be weird in actual meanings. Like, you can mix group A and B in simple sentence without much context. Example, the puppy is sleeping. A: 강아지'가' 자고 있다. B: 강아지'는' 자고 있다. Both means same actually. But to show you the diffence, if you use group A then it actually is focused on the subject. Let's say, someone asked "who is sleeping there?" Then you can answer 'the puppy' is sleeping. So what important here is the puppy which is the subject of sentence. Then the answer goes like: 강아지'가' 자고 있다. But group B is focused on topic(verb) more. So let's say "what is the puppy doing?" Then you can answer the puppy is 'sleeping'. So what important here is sleeping which is the topic. Then you say : 강아지'는' 자고 있다.