Learn Dutch: Ik dank u voor de koffie. - I thank you for the coffee.

Ik dank u uses the verb danken (to thank). It is grammatically correct but sounds very formal and a bit stiff for a simple cup of coffee. In everyday life, a Dutch speaker would usually just say dank u wel voor de koffie or bedankt voor de koffie.

Can I say ik dank je if I'm speaking to a friend?

Grammatically, yes, because je is the informal word for "you." However, because the phrase ik dank... is already quite formal, combining it with the informal je sounds unnatural. For friends, it is much better to simply say bedankt voor de koffie (thanks for the coffee).

Does the verb danken always take the preposition voor?

Yes! Just like in English, you thank someone for something. You will use voor whether you are using the verb danken, or more common everyday expressions like bedankt voor (thanks for) and dank u wel voor (thank you for).

Is dank here a verb or a noun?

In this sentence, it is a verb. It is the "I" form (first-person singular) of the verb danken (to thank). You will also see dank used as a noun meaning "thanks," like in the common phrase met dank aan (with thanks to).