Moderator note: multiple threads merged to create this one. If you need an indirect object, use leur (= "to them, for them"). It is invariable. If you need a possessive, use leur (= "their X") or leurs (= "their Xs"). Use the singular if the thing they own is singular, and use the plural if the things they own are plural. If you need more detail, or examples, keep reading! See also the following thread on the Français Seulement forum: leur / leurs. Can anyone help clarify the rules about using leur or leurs? Is it really as simple as singular/plurals?