Learn Croatian: Gdje su mačke? - Where are the cats?

The singular word for a cat is mačka. In Croatian, regular feminine nouns end in -a. To make them plural, you change that final -a into an -e. This is why the plural form is mačke.

Why do we use su instead of je?

The word su means 'are' when talking about 'they'. Since cats are plural, we must use su. If we were asking about a single cat, we would use the singular verb je and ask Gdje je mačka?.

Is mačka always a feminine noun?

Yes, grammatically mačka is always a feminine noun, and it is the default word for a cat of any gender. If you need to specify that a cat is male, there is a separate masculine word, mačak.