If you're learning French, there is a significant difference with English in that adjectives change according to masculine/feminine and singular/plural. So in your example, the gender explain the difference :
- masculine singular : "he is handsome" translate to "il est beau"
- feminine singular : "she is handsome" translate to "elle est belle"
- masculine plural : "they are handsome" translate to "ils sont beaux"
- feminine plural: "they are handsome" (with exclusively female subjects1) translate to "elles sont belles"
You may also find "bel" as a male singular form, when the adjective appears before the vowel-starting noun. For example, you will say "c'est un bel homme" and not "c'est un beau homme". If you're a starter, do not focus on this, but you may read it here and there...
Note 1: This grammatical rule known as "Le masculin l'emporte sur le féminin" raises discussion in France, as some consider it implies gender discrimination.
bel
to be used for male gender when the next word starts with a vowel sound:un bel arbre/un bel homme
. – njzk2 Jan 21 '15 at 19:38