Q. 한국어에 언제 ''나는''''제가''''저는'''을 씁니까?
they are subjects.
'나는' is similar to '저는', but '저는' is an honorific form.
when you talk to older people or many people(not 1:1) or when you meet someone at the first time(which means if you're not familiar with someone), you'd better use that form.
'제가' is also a polite expression,
제 = 저, but we don't use '저가' or '제는'.
try to search 은/는.이/가