"Drink less milk."
Translation:Bebe menos leche.
It's the command form of the verbs. The tú commands are a bit weird. The command for tú in the AFFIRMATIVE is the 3rd person form of the verb ("bebe"), the NEGATIVE uses the subjunctive ("NO bebas"). The command of usted and ustedes always uses the subjunctive. http://spanish.about.com/od/verbmoods/a/direct_commands.htm
Affirmative: bebe menos agua, Negative: no bebas más agua. The affirmative of the "tú" form uses the 3rd person present tense, the negative uses the subjunctive. http://spanish.about.com/od/verbmoods/a/direct_commands.htm
It looks to me like a lot of people are confusing the subjunctive with the imperative (that is what I did). I believe rspreng's response is correct (I'm an english speaker learning spanish so I'm no expert).
'Drink less milk' sounds like a command to me.... Look up the imperative of beber (501 spanish verbs). You only use bebas as a negative 'no bebas'. The subjuctive is different.
I'm guessing it would be correct to say something like 'quiero que tu bebas menos leche', but I think if I'm ordering you to do it I would need the imperative 'bebe menos leche!! (you familiar)' or 'beban menos leche!' (I order you all to drink less milk) or 'no bebas menos leche!' (don't drink less milk).
The first sentence I'm asking someone to drink less milk. He or she may or may not do it. The last three statements I'm ordering someone or a group of people to do something and I expect to be obeyed.
You are right about beba usted, but there is a form bebé in the imperative for the verb beber for the second person pronoun 'vos' (http://www.wordreference.com/definicion/vos). See http://www.wordreference.com/conj/ESverbs.aspx?v=beber for a complete conjugation of the verb.
For the vast majority of verbs, the affirmative tú command is identical to the third person singular of the present indicative:
¡Habla en voz alta! Speak up! ¡Come con nosotros! Eat with us! ¡Vive bien! Live well!
All commands except affirmative tú and vosotros are based on the subjunctive.