Modal Verbs

Modal verbs incorporate a special meaning component expressing ability, obligation/ advisability, necessity, possibility, and permission/request.

In French they are:

pouvoir, savoir: to express ability.
(also to express permission/request)

Les astronautes ont pu aller sur la lune.
(The astronauts were able to go to the moon.)

Votre enfant est trop jeune pour savoir lire.
(Your child is too young to know how to read.)

falloir: to express necessity:

Il faut me payer tout de suite car j'ai besoin d'argent.
(You must pay me right away because I need money.)

devoir: to express primarily obligation/advisability:

On doit toujours être poli.
(One must always be polite.)

See the individual topics for details.