나 and 저 are used for informal and formal situations respectively. 내가 - informal, talking with friends, younger people 제가 - formal, taking to older people, authority 나는 - same, but using the "는" distinguishing marker 저는 - same, but using the "는" distinguishing marker Here is more about 는 은 http://www.talktomeinkorean.com/lessons/l1l9/