누가 가르칩니까?

선생님이 학생을 가르칩니다.

Here, we are using 이 to identify the subject (the teacher): "The teacher is (이) teaching the students."

선생님은 친절해요.

Here we are using 은 to describe the subject. "The teacher is (은) kind."

누가 오빠입니까?

이 사람이 우리 오빠입니다.

Here we are using 이 to identifying the subject: "This person is (이) our older brother."

우리 오빠는 잘 생겼어요.

Here we are using 는 to describe our subject: "Our brother is (는) handsome."

누가 왔어요?

친구가 왔어요.

Here we are using 가 to identify the subject: "My friend has (가) arrived."

친구 이름은 김민기예요.

Here we are using 은 to describe the friend's name: "My friend's name is (은) Mingi Kim."

어디가 화장실에요?

저기가 화장실이에요.

Here we are using 가 to identify a direction: "Over there is (가) the bathroom." 

여기는 화장실이 아니에요.

Here we are using 는 to describe a place: "This place is (는) not the bathroom."

From doing the translations above, it appears that the typical use cases for the following are:

- 이 for identifying the subject 

- 가 for identifying the subject

- 은 to describe the subject

- 는 to describe the subject

To elaborate a bit further...

- 이/가 are used in cases when it is important to emphasize/centralize the subject to make the point of the statement. 

- 은/는 are used in cases when you do not need to emphasize/centralize the subject to make the point of the statement, but rather just describe the subject

I hope that helps!  : )

I'll be happy to help some more once I set up my teaching profile to become a community tutor! 

Cheers from NYC!