저, 저희, 당신, 그분들 these are all for to be polite :) so when you talk to others who are older than you(or other people who provide you something) you should use 저, 저희, 당신, 그분들. e.g: 선생님, 저는 오늘 아파서 학교에 못 갑니다. (Seon Saeng Nim, / Jeo Neun / O Neul / A Pa Seo / Hak Gyo E / Mot Gam Ni Da.) Teach, I'm sick so I can't go to school today. In addition 네가 is correct form to write something but when we talk to others nomally Korean people say '니가'. because if we use 네가 while you are talking the listener will get comfuse between 내가 and 네가. And 저가 x 제가 o / 너가 x 네가(to write) 니가(to talk)/ 나가 x 내가(both) 제가(to be polite). It seems taken same reason as She / her / her, I / my / me. So don't think seriously about it :) I will be happy if you are saticefied it. :D 공부 열심히 해! Study hard! [Gong Bu / Yeol Sim Hi / Hae!]

You will hear people in 서울(and 경기도) say 너가 - it's the 서울/경기도 dialect. In 네가/제가/내가, 네/제/내 doesn't use the possessive form. It's just a rule to use 네/제/내 with -이/가, instead of 너/저/나.

first of all.. '니' is from Kyungsang dialect so far i know.. 내(my) is short form of 나의, 네(your) is short form of 너의, when YOU use , 저/나/너 use with 이/가, you should use '제가, 내가, 네가'... if you use '저가, 나가, 너가,' it sounds like 'countryside dialect' i don't know which area... anyway..'제가, 내가, 네가' is special grammer in korean..

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I know this is an old question but thought you might want to know the reason why the forms are 네가/제가/내가 and not 너가/저가/나가 in modern standard Korean. Hailey's answer below is right. The irregular forms 네가/제가/내가 are not related to the possessive forms. The possessive forms are just phonetic simplifications of 너의/저의/나의, respectively. The '으', which is the weakest vowel, gets dropped and then the remaining '이' merges with the preceding vowel. The subject forms 네가/제가/내가 are that way for historical reasons. The subject marker '-가' is actually fairly new to Korean. A few hundred years ago, it didn't exist and there was only '-이' to mark the grammatical subject of a verb. The problem was that when '-이' followed a word ending in a vowel, it was not so clear. So over time, the language evolved and people started adding what was probably originally an emphatic marker '-가' after '-이' to make the subject marked more clearly. After a few hundred years, the modern rule for '-이' after words ending in a consonant and '-가' after words ending in a vowel became the norm. 네가/제가/내가 are all just holdovers from this process and are thus irregular in modern Korean.