The basic rule
Object pronouns (me, te, le, la, lui, nous, vous, les, leur) go before the conjugated verb in French. English puts them after the verb: I see him. French moves them to the front.
Je le vois.
I see him.
Elle me parle.
She speaks to me.
Nous les connaissons.
We know them.
In negative sentences
The negation ne … pas wraps around pronoun and verb together: ne goes before the pronoun, pas after the verb. There is no English-style do – just ne and pas.
Je ne le comprends pas.
I don't understand him.
Elle ne me connaît pas.
She doesn't know me.
Nous ne leur parlons pas.
We don't speak to them.
With an infinitive
With two verbs (for example aller, pouvoir, vouloir + infinitive) the pronoun goes before the infinitive, that is, before the verb it belongs to. English does the opposite: I want to see him.
Je vais le faire.
I am going to do it.
Tu peux m’aider ?
Can you help me?
Je ne veux pas le voir.
I don't want to see him.
In the passé composé
In the passé composé the pronoun goes before the auxiliary, not after the participle as in English (I have seen him). In the negative, ne … pas wraps around the auxiliary together with the pronoun.
Je l’ai vu hier.
I saw him yesterday.
Elle m’a répondu.
She answered me.
Je ne les ai pas invités.
I didn't invite them.
When a direct pronoun (le, la, les) comes before avoir, the participle agrees with it: la adds -e, les adds -s. With lui and leur the participle does not change.
Marie ? Je l’ai vue.
Marie? I saw her.
Les clés ? Je les ai trouvées.
The keys? I found them.
Marie ? Je lui ai parlé.
Marie? I spoke to her.
