leur
to them (indirect object pronoun)
for them (indirect object)
Je leur ai envoyé un message tout à l'heure.
I sent them a message a little while ago.
their (possessive determiner; used before a singular noun)
Notes
GENERAL INFO
Leur means "their" when it's a possessive adjective, or "to them"/"them" when it's an indirect object pronoun.
USAGE
As a possessive adjective: *C'est leur chien.* (It's their dog.) *Ce sont leurs livres.* (These are their books.)
As an indirect object pronoun: *Je leur donne un cadeau.* (I give them a gift.) *Elle leur téléphone.* (She calls them.)
TRAPS
The possessive adjective leur agrees in number with the noun it modifies (e.g., *leur* livre vs. *leurs* livres). However, the indirect object pronoun leur ("to them") is always singular in form and invariable, even if it refers to multiple people.
