Can you tell me difference between 나, 나는, 내가, 니가, 저요 and 저는? And when to use like informal and formal usage? Thanks in advance
First, 나, 나는, 내가, and 네가 (colloquially pronounced or informally written as 니가) are all informal, while 저요 and 저는 are both formal/humble.
To understand the rest of the explanations, I suggest reading this short post about particles.
나 means “I/me.” 나는 is a combination of 나 and the topic particle -는. 내가 is a combination of 나 and the subject particle -가.
To explain 니가, I will give you a little bit of background information first. The informal word for “you” is 너. 너 combined with the subject particle -가 creates 네가, which you can think of as the “you” equivalent of 내가. However, the pronunciations of 네가 and 내가 sound almost the same. To avoid confusion, people often pronounce 네가 as 니가 in speech or write it that way in informal writing (like texting). 네가 is technically correct, but 니가 is commonly used in speech instead.
저는 is the combination of 저 (”I/me,” formal/humble word) and the topic particle -는. You can think of it as the formal/polite/humble equivalent of 나는.
저요 is simply a polite way of saying “I/me” as a single phrase (not within a sentence). -요 is a polite ending that can be attached to the end of an utterance or a noun. So 저요 is the combination of 저 (polite “I/me”) and the polite ending -요. I will give a few examples below to show how 저요 can be used, with only the necessary part in Korean so that you can fully understand the example.
A: Who ate the last piece of cake?B: 저요. (Me.)
A: Who is the project manager?B: 저요. (Me.)
Informal and Formal Usage
Informal speech (also called 반말) should only be used when speaking to children, childhood friends, very close friends, yourself, or people who have given you permission to speak informally. Thus, it is a good idea to make polite speech your “default” form of speech so as not to accidentally speak informally to someone older, a stranger, or an authority.
Polite speech typically uses the humble words for “I” and “we,” 저 and 저희, respectively. Also, sentences should end in -요, the polite speech marker. This level of speech can be used with most strangers, teachers, peers, colleagues, acquaintances, and friends.
저 and 저희 are humble/polite/formal in the sense that they are showing that the person you are speaking to is above you and you are below them. It is, in general, very important to use these in polite speech, but the informal words 나(I/me), 너 (you), and 우리 (we) can also be used with the polite speech marker -요.
Finally, the deferential form is a form of speech that is more extreme than polite speech and also shows that the listener is at a higher level than you. It is used when speaking to someone who is your senior in rank or who is much older, respected, or honored. You would use deferential speech with teachers, professors, your boss, and the elderly. It is also used when addressing an audience or group (including on television in news broadcasts) to be polite. 저 and 저희 (not 나 or 우리) must be used in the deferential form. You may have already seen or heard the verb endings that characterize the deferential form: -습니다, -ㅂ니다, -습니까, and -ㅂ니까.
You may have noticed that I did not mention a polite word for “you.” This is because in general, it is impolite to say “you” to the listener unless you are speaking informally. The norm is to use the listener’s name, title, or name and title as the subject instead of “you” in the polite and deferential forms.