You add the definite article if your talking about some specific milk. If you leave the definite article you are talking about milk in general:
- Ik drink de melk (die in dit glas zit) - I drink the milk that's in this glass
- Ik drink melk - I drink (some) milk
I think that's obvious and identical to English. The more interesting question to me is whether "Ik drink (de) melk" refers to RIGHT NOW - which in English would be "I am drinking (the) milk - or refers to a general fact/habit, like English I drink milk (all the time/ at lunch).
Het and de refer to the gender of the nouns, which are non-existent in English. Basically Het is the neutral singular term, and De is both feminine, masculine and all the plural terms. The use of Het depends on certain words, which can sometimes be "neutral-gendered"
Ik drink - I drink Jij drinkt - You drink Hij drinkt - He drinks Zij drinkt - she drinks Wij drinken - we drink Jullie drinken - you drink (plural) Hij drinken - they drink
It's important to say hij drinkt if your talking about 'Him' and Jij drinken if talking about 'they'