I'm new to this website but I've had lessons in german before and "Sie" means Her, You (formal) and They. It would be Sie(fem) trinkt, Sie trinken and Sie trinken. I just think this should be made more clear, some, if they haven't read the Tips and notes in detail would have no idea of this. I don't know if it's made clearer in the next lessons of something.
The verb 'to drink' is 'trinken'. This is the infinitive form. The verb is conjugated with just the root 'trink-'.
The regular verb conjugation is: ich (-e), du (-st), er/sie/es (-t), wir (-en), ihr (-t), sie (-en).
So: ich trink-e, du trink-st, er/sie/es trink-t, wir trink-en, ihr trink-t, sie trink-en.
THUS, trinkt with 'er/sie/es' and 'sie(plural)' and trinke with 'ich'