축약형 form of noun
나 ＋가 ⇨ 내가 I (informal) how does 나 became 내
저 ＋가 ⇨ 제가 I (polite) how does 저 become 제
너 ＋가 ⇨ 네가 you (informal ) how does 너 become네
누구＋가 ⇨ 누가 who how does 누구 become 누

Question

축약형 form of noun
나 ＋가 ⇨ 내가 I (informal) how does 나 became 내
저 ＋가 ⇨ 제가 I (polite)     how does 저 become 제
너 ＋가 ⇨ 네가 you (informal ) how  does 너  become네
누구＋가 ⇨ 누가 who                how does 누구 become 누

Answer

In this case, 내/제/네 is not short for 나의/저의/너의. It's just that 나/저/너 becomes 내/제/네 when used with 가. 

It's a rule that exists without any reason, or at least I can't work out. In 경기도 and 서울, people will say 너가 and 니가(less common). In other cities, people say 니가 in general.

Answer

나+의 = 내
저+의 = 제
너+의 = 네
all three of them has the same function as possessive expression. 
my/formal my/your

누구 doesn't become 누

Answer

감사합니다 하지만 난 여전히 혼란스러워요.

Answer

감사합니다 하지만 난 여전히 혼란스러워요.

Answer

잘 하셨습니다. ^^