"Du trinkst Milch."

Translation: You drink milk.

[RuiLima]

Ich trinke, du trinkst, er/sie/es trinkt.

[ankit_bansal]

When is trinken used?

[spanishpaula]

It is used for Wir and Sie (we and they)

[Dewish-siss]

Sie=they ?

[RuiLima]

Sie=you in a formal way, i.e., Can I help you? - Was wünschen Sie? (with capital letter). This is used when addressing to someone you've never met before or addressing to the Prime Minister for instance.
sie (non capital letter) is the 3rd person plural and it means they.

[Sariwes]

How can I know if they are talking about "drink" or "are drinking?

[lilybett2k]

In German, they don't have any difference between those two. Just pick the one that sounds best in English.

[anandmonty]

What is the difference between 'Du trinkt Milch' and 'Du trinkst Milch' ?

[christian]

"Du trinkt" is ungrammatical.

http://goo.gl/pA0pZ

[Ttidman]

what is the difference between Trinke, Trinkst, and Trinkt?

[christian]

Make sure you always read the tips.

http://www.duolingo.com/skill/de/Basics-1

http://www.dartmouth.edu/~german/Grammatik/PresentTense/Present.html

[rdaigle1209]

How do you properly pronounce Milch?

[christian]

/mɪlç/

http://en.wiktionary.org/wiki/Appendix:German_pronunciation

http://www.pauljoycegerman.co.uk/pronounce/consonch.html