Just to simplify. 나 = neutral 내가 = subject, emphasis on that it's YOU. 나는 - > 난 = also subject, kinda, but 는 is called a topic particle. 나를 - > 날 = Object same thing for the polite version: 저 제가 저는 - > 전 저를 - > 절 Here are some audio lessons on the different particles: 은/는, 이/가 http://www.talktomeinkorean.com/lessons/l1l9/ 을/를 http://www.talktomeinkorean.com/lessons/level-2-lesson-2-%EC%9D%84-%EB%A5%BC-object-marking-particles/

1. 난 and 전 are short for 나는 and 저는 respectively. 2. 나 means "I" in general and 저 is the humble form of 나 3. 나 or 저 is just a pronoun, but 나는 or 저는 can be a subject in a sentence. 4. The humble form 제가 of 내가 is also used as a subject, often used to emphasize "subject itself" We've got several speech styles depending on formality, politeness and intimacy with a listener, but the humble forms are used for strangers or older persons than you. e.g. I ate the apple. 내가 그 사과를 먹었어요. (You did, not others) 나는 그 사과를 먹었어요. (more interested in what you did) \^o^/