In Korean, 니가 (pronounced nee-gah) is used to refer to someone non-formally in the second-person as the subject in a sentence.

When you want to say "You" in non-formal language, the word is 너 (nuh). And if you wanted to use it as the subject, you would add the subject marker 가 (gah) to it, to form 너가 (nuh-gah). But that's not actually standard Korean, which uses 네가 (neh-gah).

BUT the problem with using 네가 is that we also have the word 내가, which is how you refer to yourself non-formally as the subject in a sentence. And 내가... is also pronounced (neh-gah), basically. So, to clear up the confusion, instead of using 네가, people started using 니가 instead.

In Korean, 니가 (pronounced nee-gah) is used to refer to someone non-formally in the second-person as the subject in a sentence.

When you want to say "You" in non-formal language, the word is 너 (nuh). And if you wanted to use it as the subject, you would add the subject marker 가 (gah) to it, to form 너가 (nuh-gah). But that's not actually standard Korean, which uses 네가 (neh-gah).

BUT the problem with using 네가 is that we also have the word 내가, which is how you refer to yourself non-formally as the subject in a sentence. AND THEY BOTH SOUND BASICALLY EXACTLY THE SAME. So, to clear up the confusion, instead of using 네가, people started using 니가 instead.