*저는, 나는 and 내가" - 나: I, the first person, used in plain speech. - 저: I used in respectful speech. It has a sense of lowering oneself, to show respect to the other person. When these are combined with certain particles, they sometimes change form. The changes are most of the time to make the pronunciation smooth. - 나는 (I am), 나를(me), 나도(I also), 나라면 (if it's me) ... - 내가 (I am), 내게(to me), 내(my), ... - 저는, 저를, 저도, 저라면, ... - 제가, 제게, 제, ... You can check grammar books and sites to learn them over time with their usage examples. It's not something you can just quickly memorize.