Questions & Answers about El metro es muy rápido.
Why do we use es instead of está to say "is" here?
Why do we use muy instead of mucho for "very"?
Does the word rápido change if we describe a feminine word?
Yes, it does! Adjectives in Spanish must match the gender of the noun they describe. Because el metro is a masculine noun, we use the masculine ending -o to make rápido. If we were describing something feminine like la bicicleta (the bicycle), it would become rápida.