"The duck is drinking milk."
Translation:De eend drinkt melk.
You have to memorize if it's de or het for every noun. But sometimes, it follows this rule, if it can be given a gender, like a person, it is a "de" noun. But if it can't, like a book, it is a "het" noun. Keep in mind that this rule is only SOMETIMES applied, so it is best to just memorize the whole thing. :P
EDIT: For nouns that already have a gender, like "meisje", "het" is used.
"The duck is drinking milk" is to be translated as "De eend is melk aan het drinken" "De eend drinkt milk" is "The duck drinks milk"
Actually, the English present continuous ("the duck is drinking milk") can often be translated with the Dutch present simple ("de eend drinkt melk"). The use of tenses is different in the two languages: the Dutch present continuous ("de eend is melk aan het drinken") isn't used that much and can almost always be replaced by the present simple, while the English present continuous is very common.