When there are two objects in an active sentence, there are two possible active sentences and two possible passive sentences.
possibility 1: The professor explained the students the exercise.
possibility 2: The professor explained the exercise to the students.
There are two objects in each of the following sentences:
object 1 = indirect object: the students
object 2 = direct object: the exercise
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||explained||the students||the exercise.|
active sentence – possibility 2
|subject||verb||direct object||indirect object|
|The professor||explained||the exercise||to the students.|
Each of the objects (direct and indirect) can be the subject in the passive sentence.
passive sentence – possibility 1
|The students||were explained||the exercise.||(by the professor).|
passive sentence – possibility 2
|The exercise||was explained||to the students||(by the professor).|
Possibility 1 is sometimes called Personal passive.