이, 가 is only subject, remember that. You use 이 when a word ends with a consonant ( example = 집 + 이 = 집이 = the house ) ; 가 when ends with a vowel instead (example = 코끼리 + 가 = 코끼리가 = the elephant). 은,는 can be also subject in some circumstances but it's never 100% that..because... the literal meaning of 은,는 is "about" or "regarding" , something like that...so when you say "나는" it coincides with the subject but it's literally "about me" , "regarding me", "as for me" ...so we could say that 은,는 is "argument"...