"The slow cows drink water."
Translation:De langzame koeien drinken water.
Each noun uses either "de" or "het". In most cases, you will have to memorize which article to use for every new noun you learn. There are some rules that makes things easier though: for example, plural nouns (like "koeien") always use "de". There is some more information here.
I see that you have learned some French: it is similar to the French articles "le" and "la" (although in Dutch, it doesn't quite correspond to masculine/feminine nouns).