The article must match the grammatical gender of the noun. In Flemish like in German the three genders masculine, feminine and neuter are about equally common. In Standard Dutch, masculine and feminine have been merged into the so-called common gender, which is therefore twice as common as neuter gender.
- het paard, de paarden - neutral gender
- de vogel, de vogelen - common gender
This is why you should always learn the definite article along with every Dutch noun.
There are classes of nouns that are more likely to get one gender or another. These classes can be defined semantically (metals, male people/animals, female people/animals, fruit, etc.) or by other criteria such as endings. But there are no strict rules, and sometimes the rules contradict each other anyway. Het meisje is one such case, in which the (neuter gender) diminutive ending happens to have won against the natural gender of girls .
For most classes, such as birds, there isn't even a specific rule and the situation is just chaotic.