"Are you drinking?"
the sentence is "are you drinking?" which obviously that's a question
"du trinkst" is a statement translating to "you are drinking" but the sentence is a question
that's why in german they put "you, I,we" last if its a question so "trinkst du" is correct because it isn't a statement like the other one hope this helps!
du trinskt is not in the form of a question for German, in english it means "you drink" or "you are drinking" which is a statement, but also in english you could just change your inflection to make it a question, but not in german, you have to change the wording around so it would be "drink you" or "drink are you" which in english is always a question, it's just worded weirdly, as a way to think about it imagine how yoda would ask the question
I know this is an old question, but I'm going to apply my understanding of German grammar to answer this question for anyone reading this question at a later date.
Basically, when asking a question, you should switch the position of the verb and the subject.
For example: You are drinking = Du trinkst || Are you drinking? = Trinkst du?