(iv) An intransitive verb may be changed into passive (i) when it takes a cognate
object in the active (ii) when it is a preposition or group verb.
The battle was fought at this place.
We are laughed at by them.
(v) Double passive should be avoided.
The evil was sought to be corrected. (incorrect)
They sought to correct the evil. (correct)
(vi) Factitive object and adverbial object can not be used in the passive.
Active: They made him king.
Passive: He was made king by them.
In case of complex sentence, both principal and sub-ordinate clause should be changed into passive.
Active: I know that he did it.
Passive: It is known to me that it was done by him.
Passive: That it was done by him, is known to me.