When using the Voice Conjugator to change the voice of a sentence from active to passive, you might find out that there are active sentences that don’t have a passive counterpart.
Intransitive verbs like ‘rest’, ‘dine’, ‘die’, ‘fall’, etc. cannot be passivized at all.
The vase fell > *The vase was fallen
Many transitive verbs take a non-obligatory complement. When a verb occurs without its complement, it cannot be passivized.
He ate the bread > The bread was eaten by him He ate > *[??] was eaten by him
Some verbs taking a direct object (sometimes with other complements following it) do not passivize their direct object.
This resort boasts the best beaches > *The best beaches are boasted by this resort This behavior becomes you > *You are become by this behavior They want him to leave > *He is wanted to leave by them They like this beer cold > *This beer is liked called by them
Many verbs taking a clause as complement (marked with brackets in the examples below) do not passivize their clausal complement.
We complained [that he had left] > *That he had left was complained by us She finished [painting the house] > *Painting the house was finished by her They expected [to receive more help] > *To receive more help was expected
You can read more about passivization in our article about the Voice Conjugator.