"The horses are fast."
Translation:La ĉevaloj estas rapidaj.
Rapidas doesn't mean "is fast". Instead, it means "hurries". In case a single word definition doesn't make sense, I mean "hurries" as in "makes an effort to go faster than normal". In the wise (paraphrased) words of Salivanto, "if it is an adjective in English, it is almost always an adjective in Esperanto".