What's the difference between Ni, Du and Dig.
There is some other words like
"Om" and "På"
"Den" and "Det"
"Som" and "Så"
"Ska" and "Till"
and dig is for you ,like me is for I .. I like you - jag tycker om dig, You like me - du tycker om mig
Du is singular (like "thou"), ni is plural ("y'all"). Se this thread that explains why "ni" is not a polite pronoun, with some discussion on the subject.
I'll have to get back to you on the other words (unless someone else explains it to you in the meantime), as I am just about to leave my work.
So, allow me to say that the translations of these individual words are very contextual, and without context they lose much of their use. Regardless of that, I'll try to give you a broad explanation.
Om and På
Essentially, they mean "about" or "if" and "on" respectively, BUT the translation of prepositions is highly irregular, and often has to be learned by heart.
Den and Det
They both mean "it", but as a general rule "den" is used for en-words and people, while "det" is used for ett-words and things. At the start of a sentence, where there is a formal subject (like "it" in "it's raining") it's always "det" though. And in some cases, den/det translate to "that" or "the".
Som and **Så
Som is used where English uses a relative pronoun ("he *who enters here..."), and to mean "like" to make comparisons. Så has a lot of uses, and wiktionary explains them quite well.
Ska and Till
Ska is one way used to form future tense. There's a bit more explained in the future tense lesson in the tree. Till means "to", but as with om and på, its use is somewhat unpredictable in translation.
Dig (you (singular objective case)) is like honom (him), they are both objects of a sentance (what the verb acts out on)
The difference between den and det is the same as the difference between en and ett. Which is to say, the only difference is the gender of the word they're pointing to.
E.G. I like that book, jag tycker om den boken
I live in that house, jag bor i det huset