Question

What sentences we cannot change to passive?

Answer 1

The sentences, having intransitive verbs, cannot be converted into Passive Voice. The only reason for this is that such a sentence has no object. When a sentence has no object, it cannot be changed into Passive Voice. For changing a sentence into passive voice, the sentence must have clear known object.

Answer 2

we cannot change perfect continuous of any tense as well as we can't change future continuous tense into passive voice.