Learn Spanish (Spain): La casa de los gatos es pequeña.

Because there is more than one cat! In Spanish, when a noun becomes plural, the word for 'the' has to change too. The singular masculine el becomes the plural masculine los.

How do you make a word like gato plural?

For nouns that end in a vowel (like the 'o' in gato), you just add an -s to the end. That gives us gatos (cats).

Wait, if there are plural cats (los gatos), why is the word for small (pequeña) singular?

Because pequeña is describing the house (la casa), not the cats! Since there is only one house, and casa is a feminine noun, the adjective must be feminine and singular (pequeña).

Why does the sentence use es ('is') instead of son ('are') if there are multiple cats?

Just like with the adjective, the main subject of the sentence is the house (la casa), not the cats. We are saying 'The house... is small.' Therefore, we use the singular verb es.

Why is it la casa de los gatos instead of just translating 'the cats house'?