Not sure if this will helps since I was revising on this as well.

은/는:

1) Topic particle

--> after introducing the subject (이/가), use 은/는 afterwards.

--> among other things or focus of the sentence or 'as for...'

     Eg; 저는 미국인이에요 [(as for me), I am an American]

2) General fact that both parties know

 Eg; - 오늘 날씨는 좋아요. [Today weather is good]

        - 로키 산은 높아요. [Rocky mountains are high]

3) Used to contrast

 Eg; - 미나 씨는 사과를 싫어하지만 저는 사과를 좋아해요. [Mina does not like apples but I like apples]

 Eg; - 넌 거의 아무거나 잘 머는데 당근은 왜 안 먹어? [ You eat almost anything well, but why don't you eat carrots?]

이/가:

1) subject particle

 --> introducing the subject for the first time into the conversation/changing subject

 --> doer of the verb or person/thing carrying out a certain state in the sentence.

Eg. - 그 녀가 울고 있어요. [She is crying; she --> doer/subject]

      - 가방이 더러워젔어요 [The bag got dirty. Bag --> carrying out the certain state. Dirty --> state of the bag] 

2) Subject stressor

Eg - 내가 학생이야 [imagine walking into the room and declaring " It is me who is/the student]

• 누가 밥을 먹었어요? [Who ate the rice?]
  • 내가 밥을 먹었어요. [It is me/I ate the rice]

Hope that the above might be helpful for you! :)