Learn Italian: I cani sono più grassi dei gatti. - The dogs are fatter than the cats.

When you use di (than) before a noun that has a definite article, the two words merge together. Here, you are comparing the dogs to "the cats" (i gatti). The word di plus the article i combine to make dei.

Why does Italian use più grassi instead of a single word for "fatter"?

Unlike English, which often adds "-er" to the end of a short word (fat becomes fatter), Italian doesn't have an equivalent ending. To make a comparison, you always use più (more) or meno (less) before the adjective. So, "fatter" is literally said as "more fat" (più grassi).

Why does grasso (fat) end in an "i" here?

Adjectives in Italian must match the gender and amount (singular or plural) of the noun they are describing. Because i cani (the dogs) is masculine and plural, the adjective grasso changes its ending to -i to become grassi.

Could this sentence also mean "Dogs are fatter than cats" in general?

Yes! In English, we usually drop the word "the" when talking about an entire category of things (Dogs are fatter than cats). However, in Italian, you almost always need to keep the definite articles (i cani, i gatti) even when making general statements about a whole group.