When there are two objects in an active sentence, there are two possible active sentences and two possible passive sentences.
possibility 1: The professor gave the students the books.
possibility 2: The professor gave the books to the students.
There are two objects in each of the following sentences:
object 1 = indirect object: the students
object 2 = direct object: the books
An indirect object is very often a person, a direct object a thing. When a direct object is followed by an indirect one, we put to in front of the indirect object.
|active sentence - possibility 1|
|subject||verb||indirect object||direct object|
|The professor||the students||the books.|
|active sentence - possibility 2|
|subject||verb||direct object||indirect object|
|The professor||gave||the books||to the students.|
Each of the objects (direct and indirect) can be the subject in the passive sentence.
|passive sentence - possibility 1|
|The students||the books.||(by the professor).|
|passive sentence - possibility 2|
|The books||were given||to the students||(by the professor).|
Possibility 1 is sometimes called Personal passive.