As we mentioned in our last lesson, a direct object is a noun that receives the action of a verb (such as "the ball" in "I throw the ball"). On the other hand, an indirect object indicates to whom or for whom the action is done (such as "my friend" in "I throw the ball to my friend"). Just as direct object pronouns replace direct objects (e.g. "I throw it to my friend"), indirect object pronouns replace indirect objects ("I throw the ball to him/her"). There are six indirect object pronouns in French:
me (to me) nous (to us)
te (to you) vous (to you)
lui (to him/her) leur (to them)
In French, an indirect object pronoun usually replaces "à (to) + a person." Unlike direct object pronouns, which can refer to either people or things, indirect object pronouns only refer to people.
Je jette le ballon à mon amie. / Je lui jette le ballon.
I throw the ball to my friend. / I throw her the ball [or "I throw the ball to her"].
The following example contains a mixture of direct and indirect pronouns. How did the speaker know when to use which?
Il m'a dit: "Je le garde". Ben, je lui ai dit: "Écoutez, expliquez aux quatre cents personnes...”
He told me, "I'm keeping it." Well, I told him, "Listen, explain to the four hundred people...”
Cap. 24, Actu Vingtième: Vendanges parisiennes
It all depends on whether the verb in question would normally be followed by the preposition à. Garder isn't followed by à: you would say garder quelque chose (to keep something), but never garder à quelque chose. If you watch the video, you'll know from context that the speaker is referring to le fromage (cheese). So instead of saying je garde le fromage, he uses the direct object pronoun le (je le garde). On the other hand, you would say dire à quelqu'un (to tell someone), but never dire quelqu'un. Because of that à, the speaker knows to use the indirect objects me and lui.
Here are some other examples of indirect object pronouns in action:
Si la nuit me parle de souvenirs passés
If the night speaks to me about past memories
Cap. 3-4, Boulbar: New York, 6 heures du matin
Mais je te donne plus que des mots
But I give you more than words
Cap. 12, Corneille: Comme un fils
Et là, je leur ai envoyé une petite nouvelle…
And here, I sent them a little short story…
Cap. 86, Claudine Thibout Pivert: 2ème Salon du livre et des vieux papiers Mazamet
We know these are indirect object pronouns because they all replace "à + person" in the verbal expressions parler à quelqu'un (to speak to someone), donner à quelqu'un (to give to someone), and envoyer à quelqu'un (to send to someone).
As you learned in our last lesson, when a direct object pronoun is followed by a verb in the past tense (passé composé), the past participle needs to agree in number and gender with the direct object pronoun. On the other hand, you don't have to worry about agreement in the passé composé with indirect object pronouns. That's why you have je leur ai envoyé in the example above and not je leur ai envoyés or je leur ai envoyées.