Learn Portuguese (Portugal): Os leões não são tão rápidos como os pássaros.

To say "as fast as", Portuguese uses tão (as/so) + the adjective + como (as/like). Here, that gives us tão rápidos como to compare the lions and the birds.

Why is it rápidos instead of just rápido?

Even though it is inside the tão... como comparison, the adjective must still match the thing it describes. Since Os leões (The lions) is masculine and plural, the adjective becomes rápidos. The words tão and como never change.

How do you form the plural word leões?

The singular word for lion is leão. In Portuguese, most words that end in -ão change to -ões in the plural, making it leões.

Why is there an os before pássaros when the English translation just says "birds"?

In English, we drop the word "the" when talking about things in general. In Portuguese, you still need to use the definite article (o, a, os, as) when talking about a whole category or species, like os pássaros (birds).