What's the difference between "I" and "du"?
I is for when you're talking to more than 1 person (think of the US "y'all")
Du is for when speaking you a single person (Just you alone and no one else)
Exactly, I is the plural you, like you guys are walking to the store. And Du is the singular you, like in I am replying to you
Is there a familiar vs. polite form like in the romance languages? Or just singular/plural?
There is a formal "you" (De) but it is very rarely used these days
Would you ever use "den" in this context