Because the old rule was that: if the direct object of a transitive verb is put before this verb, then the verb must be declined accordingly to this object.
Ho visto la donna
(the object is after); lei è la donna che ho vista
(the object is represented by the relative pronoun che
, so it's before).
Ho colto le rose. Queste sono le rose che ho colte.
This rule sounds too old-fashioned today; it's still grammatically correct (my teachers at school didn't expect us to use it, but they informed us of its existence), but nobody use these forms anymore. Today, nearly everyone says la donna che ho visto
and le rose che ho colto
BUT the rule is still used for personal pronouns, like in li abbiamo visti
, ci hai delusi
, ti ho ricordata
To tell the truth, I thought in these cases this was the only correct form, but in post n. 2, Montesacro said that both forms are possible (li abbiamo visto
, ci hai deluso
, ti ho ricordato
to a female). Mmmm... I'm not sure.
Anyway, pay attention, because the atonal pronouns like mi, ti, ci
can have two different functions: they can serve as direct objects for transitive verbs (mi=me; ti=te; ci=noi; vi=voi
; and in this case, the verb is declined accordingly); or, they can serve as indirect objects for intransitive verbs (mi=a me; ti=a te; ci=a noi; vi=a voi
; and in this case, the verb is NOT declined).
This explains the first question in this thread. Chiamare
needs a direct object (she called me
) but dire
needs an indirect object (she said to me
[Last clue - don't read if you already feel too confused
can occasionally be a transitive verb and need a direct object, with the meaning of to define
, to describe,
or to name
. So, mi ha detta...
could be possible (but incomplete) with the meaning of she named me...
, she defined me as...
Anyway, a very rare and old-fashioned use
In fact, putting the sentence "mi ha detta
" on Google, all you can find is just mistakes made by someone who should have written "mi ha detto
", except maybe a transcription of an ancient comedy where old Italian allowed this form...]