Learn Dutch: Wil je meer water? - Do you want more water?

No, this is a very common "false friend"! In Dutch, the verb willen means "to want". So wil je means "do you want", not "will you". If you want to say "will" to talk about the future, Dutch uses a completely different verb (zullen).

Why is the verb wil at the beginning of the sentence?

To turn a statement into a yes/no question in Dutch, you swap the order of the subject and the verb. The statement Je wil (You want) becomes the question Wil je? (Do you want?). Notice that Dutch doesn't use a helper word like "do" to make questions!

I learned that the "you" form of verbs usually ends in "-t". Why isn't it wilt je?

When you swap the verb and the subject jij or je (you) to ask a question, the verb drops its -t ending. For example, Je drinkt (You drink) becomes Drink je? (Do you drink?). Because of this rule, the question form here is always Wil je?.

How would I ask this question if I wanted to be more polite?

Je is the informal word for "you". If you are offering water to someone older, a customer, or a stranger, you should use the formal "you", which is u. The formal version of this sentence is Wilt u meer water?. Notice that when asking a question with u, the verb keeps its -t ending!