English Translation
Cats are smart.
Why is this sentence singular in Catalan, but plural in English?
I would say it is because it is referring to cats as a species. If that is the case, you can translate it to a singular. Something like saying:
- The cat is a very smart animal.
Where, even if you are not referring to a specific individual, you are still using a determinate singular noun.
All that said, you could also translate it as. “Els gats son molt astuts”, in which you are talking about all the individuals that compose a certain group, be it a concrete group that you may be talking about, or the species as a whole.
I have been thinking about this for a bit and have changed my mind a bit. Although I still think the singular is correct, it does sound a bit forced. The more natural translation would probably be (as suggested) in plural, like in English. Something like “Els gats son molt astuts”. And even maybe “Els gats son molt llestos”. To make in singular “un animal” could be added, even if, depending on the context in which is used, it wouldn’t be necessary: El gat és (un animal) molt astut".