"They are cats."
Translation:Het zijn katten.
Nope, that is correct Dutch. Dutch is not always going to be exactly like English.
The use of 'het zijn' (= they are) is a peculiarity, specific to Dutch. Here is an link that explains it well.
We use 'het zijn' followed by a noun, where the English use 'they are'. Examples:
- "Het zijn dieren." = "They are animals."
- "Het zijn aardappelen." = "They are potatoes."
- "Het zijn lieve hondjes." = "They are sweet, little dogs."
Now, when we are dealing with an adjective this formula isn't used. Instead, you'll find sentences like these:
- "Ze zijn mooi." = "They are pretty."
- "Ze zijn aardig." = "They are nice."
- "Ze zijn groot." = "They are big."