Look at these sentences in L'Impératif:
Note that in L'Impératif, the direct object pronouns le, la, l', les and the indirect object pronouns lui, leur are placed after the verb with an hyphen in between.
It's often very confusing for students to know which pronoun to use in affirmative commands.
Look at these two examples:
Nous écrivons à notre tante -> Nous lui écrivons -> Écrivons-lui !
We're writing to our aunt -> We write to her -> Let's write to her!
Tu arrêtes Paul -> Tu l'arrêtes -> Arrête-le !
You're stopping Paul -> You're stopping him -> Stop him!
Here is the rule:
- If the verb is normally followed by à + person in French (i.e. followed by an indirect object pronoun), such as in écrire à [quelqu'un] (to write to [someone]) or montrer à [quelqu'un] (to show [someone]), then you use lui or leur.
- If the verb isn't followed by any preposition (i.e. followed by a direct object pronoun), such as arrêter [X] [quelqu'un] (to stop [someone]), you use le/la or les.
Look at these examples:
Note that quite a few verbs, such as regarder and attendre, are used without prepositions in French, i.e. regarder [x] [quelqu'un]; attendre [x] [quelqu'un], whereas they have one in English (i.e to look at [someone], to wait for [someone]).
On the other hand, some English verbs without prepositions will be followed by à in French, i.e. to phone [someone] = téléphoner à [quelqu'un].
When in doubt, check with a dictionary.