"I am drinking milk."
Translation:Je bois du lait.
I believe that milk is un-countable (you can't say "a milk" its always "some milk"), like other liquids, or rice.
Even if you want to say I DRINK MILK in english, the "some" will always be implied. In french it seems you have to literally say it out loud.
I don't know if that made sense
Aside from a few exceptions (when aren't there?), all nouns need a determiner: le lait (the milk); mon lait (my milk); son lait (his/her milk); notre lait (our milk); du (de + le) lait (some milk); etc.
Saphydawg is correct - because milk is an uncountable quantity, we use the partitive article here.