I anways thought that in English cat is "it" not "he" or "she" , so "they are" sounds pretty weird for me
Does that sentence have another conotation? In portuguese, if you call someone a cat it means the person is beautiful
In what case is "кошки"? I would have said it's in a feminine noun in the accusative case, and because it's an animate plural it would follow the rules for forming genitive plural feminine nouns, but according to those rules the "а" in "кошка" should have been dropped? Or, because (as per modern grammar) the noun "кошка" is a complement (its referent is the same as that of the subject), it should be in the nominative case, and hence follow the rules for the formation of nominative plural feminine nouns? That still not does not explain how this noun is formed. Or is it in another case?
It's in this nominative plural case. The tips & notes explain how it is formed.
Ah, I think my own notes on how the Nominative Feminine Plural is formed are wrong; the notes on Duo make much more sense. But I'm still not clear on why this is in the nominative plural – it's the object of this sentence and therefore should be in the accusative, surely?
Since you appear to be reasonably up on your grammar terminology, I think the easiest way to explain is to send you here: https://en.m.wikipedia.org/wiki/Copula_(linguistics)#Slavic
Aha, thank you! As I mentioned in my original comment, the subject-complement relationship was a hypothesis of mine, and your link confirms it - it's exactly what I was looking for.
Man/men is just an irregular plural in English, just like in Russian you have ребёнок/дети and человек/люди.
Most plurals in English you just add an S, but with a few, it's different. Man/men, woman/women, child/children. There are also a couple of words where the singular and plural words are the same, for example fish or sheep.
I have no idea what your native language is, so I have no idea if there are any irregular plurals in it, but most languages have some irregularities.
Ребята translates closer to "guys" or "friends" . Ребёнок is singular for a child. Дети is plural for children.