Normally, when you passive a verb in Latin, the following happens:
- Subject becomes a(b(s)) + ablative.
- Direct object becomes subject.
- All other constituents remain unchanged.
Note that a direct object is only that object which is directly governed by a verb (so without prepositions) and which is in the accusative.
Ciceroni parcitur a me.
No problem here.
Parcendum est Ciceroni a me.
Normally, the subject turns into a dative if you turn a verb into a gerundive + form of esse. But I believe a(b(s)) is used with a gerund if there already was another a dative present, in order to avoid confusion between two datives.
Utor lege Iuliā in Ciceronem.
This cannot be passivised, because the present forms of a deponent are already in the passive form.
Utendum est mihi lege Iuliā in Ciceronem.
This is not aequivalent to the passive of the sentence above, but at least the constituents can be transformed in a somewhat similar way:
- The ablative complement of the verb remains in the ablative.
- The prepositional phrase remains the same.
- The agent, which can now no longer be the subject, turns from an (implicit) nominative into a dative, as gerunds require.
Note that there are always exceptions, as can probably found in less formal letters and e.g. comedy.
Note also that Greek and English work differently: in those languages, the secondary complement can be turned into the subject when you passivise a verb, even when it was not a direct object. E.g. I was taught many things. Dutch works more like Latin.
Itur in Italiam.
"It was gone to Italy" → "People/they/etc. went to Italy." This is an impersonal passive, which is difficult to translate literally.
Eundum est in Italiam.
"They/someone/etc. must go to Italy."