J'ai donné le livre à mon frère - Je lui ai donné le livre.
I gave the book to my brother - I gave him the book.
Il parle à toi et à moi - Il nous parle.
He's talking to you and me - He's talking to us.
However, a few French verbs and expressions* do not allow a preceding indirect object pronoun, and what to use instead depends on whether the COI is a person or a thing.
COI = Person
When the indirect object is a person, you must keep the preposition à after the verb, and follow it with a stressed pronoun:Je pense à mes sœurs - Je pense à elles.
I'm thinking about my sisters - I'm thinking about them.
Wrong: xx Je leur pense xx
Il doit s'habituer à moi. (no change)
He has to get used to me.
Wrong: xx Il doit m'habituer xx
Fais attention à ton prof - Fais attention à lui.
Pay attention to your teacher - Pay attention to him.
Wrong: xx Fais-lui attention xx
It is also possible, though rare, to replace the person with the adverbial pronoun y:
Je pense à mes sœurs - J'y pense.
Il doit s'habituer à moi. - Il doit s'y habituer.
Fais attention à ton prof - Fais-y attention.
COI = Thing
When the indirect object is a thing, you have two equally acceptable choices: You can either keep the preposition à as above but follow it with an indefinite demonstrative pronoun, or you can replace the preposition and indirect object with y:Je songe à notre jour de mariage - Je songe à cela, J'y songe.
I'm dreaming about our wedding day - I'm dreaming about it.
Wrong: xx Je lui songe xx
Fais attention à la leçon - Fais attention à cela, Fais-y attention.
Pay attention to the lesson - Pay attention to it.
Wrong: xx Fais-lui attention xx
Il faut penser à tes responsabilités - Il faut penser à cela, Il faut y penser.
Think about your responsibilities - Think about them.
Wrong: xx Il faut lui penser xx
*French verbs and expressions that don't allow a preceding indirect object pronoun
en appeler à to appeal to, addressavoir affaire à to have to deal with
avoir recours à to have recourse to
croire à to believe in
être à to belong to
faire allusion à to allude to
faire appel à to appeal to, address
faire attention à to pay attention to
s'habituer à to get used to
penser à to think of, about
recourir à to have recourse to
renoncer à to give up, renounce
revenir à to come back to
rêver à to dream of
songer à to think, dream of
tenir à to be fond of, care about
venir à to come to
