"니" doesn't have a particular meanings. But, It makes the sentence including "니" an interrogative sentence. For example, 너는 공부하다. this sentence means that "you study" However, "너는 공부하니" this sentence means that " Do you study?" If you have more questions, you can contact to me by sending a message.

Lol that's wierd xD i started wondering that when they said "얌마 니 꿈이 뭐니?" In a hiphop song sang by boys xD well. It doesn't matter.

use 니 at end of things for like a cute meaning XD maybe only say this if you are a girl and you like the guy, and dont over use or it will be so weird 뭐 하니 <<< what are you doing 어디니 <<< where are you its really easy to tag to verbs with the vowel ending but not consonat(: so see more like 뭐 사니 진짜 자니 :( okey? okay!

Oh ok ^_^

I think it's just an informal way/simple way to make a sentences...it used when we talk with same year friends or someone younger or really close friends :D