Two positions
Unstressed pronouns (lo, la, mi, ti, gli, ci …) do not follow the verb the way English object pronouns do ("I see him"). With a conjugated verb they stand before it, as a separate word. With an infinitive they are attached to the end. The imperative has its own rule, more on that later.
Ci aspettano.
They are waiting for us.
Preferisco farlo domani.
I prefer to do it tomorrow.
Before the conjugated verb
The pronoun stands right before the conjugated verb. In a negative sentence non comes first.
Ti chiamo domani.
I'll call you tomorrow.
Gli parlo spesso.
I often talk to him.
Non ti capisco.
I don't understand you.
In the passato prossimo the verb has two parts. The pronoun goes before the auxiliary (ho, hai, ha, sono …), not before the participle.
Ti ho chiamato ieri.
I called you yesterday.
Non le ho risposto.
I didn't answer her.
Attached to the infinitive
With an infinitive the verb loses its final e and the pronoun is attached. Verb and pronoun form one word.
Non voglio perderla.
I don't want to lose her.
Vado a trovarlo.
I'm going to visit him.
After potere, volere, dovere: two options
Attached to the infinitive
Posso aiutarti.
I can help you.
Non voglio restituirle il libro.
I don't want to give her the book back.
Before the modal verb
Ti posso aiutare.
I can help you.
Non le voglio restituire il libro.
I don't want to give her the book back.
