Pronomi Oggetto Indiretto or Pronomi Indiretti, are pronouns that answer the questions “to whom?” or “for whom?”.  They are used to express the "recipient" of an action or the recipient of the direct object.

     Ex:        I gave the book to Marco.   (subj.    verb    D.O.    Indirect Object noun)        
                   I gave the book to him.        (subj.    verb    D.O.    Indirect Object pronoun)

The following table shows indirect object pronouns in Italian:

Pronomi Indiretti (forma atonica)

mi
ti
gli
le
​Le

to me
to you
to him
to her
to you (formal)

ci
vi
gli/loro


​

to us
to you (plural)
to them

You probably noticed above that there are two forms for third person plural (loro).
The loro form is technically the grammatically proper form; however, the masculine singular form, gli, has come to be commonly used in place of loro, particularly in everyday, spoken Italian.
 
   Ex:      Do il libro a Luigi e Gino.   I give the book to Luigi and Gino.    
                                                          BECOMES....
               Do loro il libro.                     I give the book to them.          (lit. syntax: I give to them the book)
               Gli do il libro.                        I give the book to them.          (lit. syntax: To them I give the book)
 
Just like Pronomi Diretti, with the exception of loro, which follows the verb, the atonic (weak) forms of i Pronomi Indiretti precede the verb*:

Paolo mi scrive            Paolo writes to me.
Paolo ti scrive              Paolo writes to you.
Paolo le scrive             Paolo writes to her.
Paolo gli scrive            Paolo writes to him.
Paolo ci scrive             Paolo writes to us.
Paolo vi scrive             Paolo writes to you (pl).
Paolo scrive loro         Paolo writes to them.         (also, Paolo gli scrive)

*As we learned with pronomi diretti, la forma atonica (weak form) can only follow (and is then attached to) the infinitive, informal imperative, and gerund verb forms. Again, with the infinitive and gerund forms, it is possible to place the pronome indiretto either before or after the verb.

[infinitive]  
​

[imperative]

​[gerund] ​

Ho un po' di tempo domani. Quindi, posso telefonarti. 
                                        oppure       (ti posso telefonare.)                 
Dimmi la verità.
                                                   
Alfredo sta parlandoti.                                       
          oppure    (ti sta parlando.)
               

 I have time tomorrow, so I can phone (to) you.
​
​Tell (to) me the truth.
​
Alfredo is speaking to you.

Pronomi Indiretti Tonici

When speakers want to emphasize the pronome indiretto, la forma tonica (tonic/stressed form) is used. This form is usually placed after the verb, but it can also sometimes precede the verb. Whereas la forma tonica del pronome diretto, is used less frequently in spoken Italian, this form is commonly used:

      Ex:            Do il libro a lui.      I give the book to him.           [This emphasizes to whom I give the book]

Let’s look at the forma tonica (stressed) alongside the forma atonica (weak), so we can see how the forms correspond:

Atonica

mi
ti
gli
le
​Le

Tonica

a me
a te
a lui
a lei
a Lei

Atonica

ci
vi
​gli / loro

​

Tonica

a noi
a voi
​a loro

​

​  (tonica- stressed)         Racconto una storia a lei.
  (atonica- weak)             Le racconto una storia.

I tell her a story.

Using Pronomi Indiretti  in compound verb tenses

1. The weak form (atonica) of the Pronome Indiretto continues to precede the verb, even in compound verb tenses like Passato Prossimo.
2. None of the forms elide / drop the ending vowel when they precede forms of the auxiliary verb avere.

 
Let’s see some examples:

                        -Hai detto la verità a Maria?                                        -Sì, le ho detto la verità
                        -Hanno dato le foto a Gino?                                         -No, non gli hanno dato le foto.
                        -Avete raccontato la storia  ai vostri genitori?       -Sì, abbiamo raccontato loro la storia