Is 이/가 and 은/는 interchangeable?
When writing in Korean, I haven't had any issues when using 은/는 vs. 이/가, but about 80% of the time I'm honestly just guessing which one to use (and usually just use 은/는). I know they don't have exactly the same meaning, so why am I able to get away with using which ever one just "feels right"? What makes "저는 게임을 좋아해요" different from "제가 게임을 좋아해요"? Can they really be used in place of one another or am I just getting lucky? Can they both be used in one sentence?? I just need a deeper explanation on the purpose of each particle...

Question

Answer

What I learnt, 는/은 are topic marker and 이/가 are subject markers. 
Ex: Today the weather is good
Today = Topic -->는/은
Weather = Subject -->이/가
Topics and subjects can be the same word, so it totally depends on the context of your sentence to decide which marker to use.

Answer

It depends on the which pronoun you use.내/네/제 are followed by 가, i.e., 내가, 네가, 제가. 나/너/저 are used with 은/는. 우리 can be used with both, so 우리는/우리가 both are correct. Regarding meaning, they are almost same. 은/는 emphasize the pronoun sometimes. For example, '저는 게임을 좋아해요' can have a hidden meaning that someone doesn't like games, but not always.

I hope this helps. If someone is professional to teach Korean grammar, please add some comments.

Answer

It depends on the which pronoun you use.내/네/제 are followed by 가, i.e., 내가, 네가, 제가. 나/너/저 are used with 은/는. 우리 can be used with both, so 우리는/우리가 both are correct. Regarding meaning, they are almost same. 은/는 emphasize the pronoun sometimes. For example, '저는 게임을 좋아해요' can have a hidden meaning that someone doesn't like games, but not always.

I hope this helps. If someone is professional to teach Korean grammar, please add some comments.