"What do you know about the mountain?"
Translation:Mit tudsz a hegyről?
Do we have to start the sentence with "Te"? Mit tudsz is not the same as Te mit tudsz?
You may include the te at the beginning or not. Either way is grammatically OK. If you include it, I think it does add a little emphasis to the "you", as in, "What do YOU know about the mountain?" Like maybe someone else has already said something about the mountain, and now I ask someone else, "What do YOU know?" - Te mit tudsz?
In conversation te (with a slight pause after it) can be a way of getting someone's attention at the beginning of a sentence. It's kind of like "hey, ..." or "hey, you..." It's fairly abrupt - about as polite as starting a sentence with "Hey" or "Hey you" in English. So I'd say be careful that you don't say it like, Te! Mit tudsz a hegyről? unless it's somebody you're on very informal terms with.