Passive voice of imperative sentences
(command and request)
A sentence which expresses command or request or advice is called imperative sentence.

For example,

Open the door.
Turn off the television.
Learn your lesson.

For changing these sentences in to passive voice, auxiliary verb “be” is used. The word “Let” is added before sentence in passive voice. Auxiliary verb “be” is added after object in sentence in passive voice. Main verb (base form) of imperative sentence is changed to 3rd form of verb (past participle) in passive voice. For best understand read the following examples.
Examples
Passive voice of “Imperative Sentences”
Active Voice
Passive Voice
Open the door.
Let the door be opened.
Complete the work.
Let the worked be completed.
Turn off the television.
Let the television be tuned off.
Learn your lesson.
Let your lesson be learnt.
Kill the snake.
Let the snake be killed.
Punish him.
Let him be punished.
Speak the truth.
Let the truth be spoken.
Help the poor.
Let the poor be helped.
Revise your book.
Let your book be revised.
Clean your room. Let your room be cleaned.

Sentences which cannot be changed into passive voice

Transitive and intransitive verb
A verb can be either transitive or intransitive. A transitive verb needs an object (in sentence) to give complete meaning while intransitive verb does need an object (in sentence) to give complete meaning.
For example,

Transitive verb.
He sent a letter.
(Send is a transitive verb and it needs an object i.e. letter to express full meaning.)

Intransitive Verb.
He laughs.
(Laugh is an intransitive verb and it does not need object for expressing full meaning.)
e.g. Sleep, go, reach, sit, die, are examples of intransitive verbs.

Intransitive verb cannot be changed into passive voice
The sentences having intransitive verbs (belonging to any tense) cannot be changed into passive voice. The reason is that there is not any object in such sentences and without object of sentence passive voice is not possible.

A sentence can be changed into passive voice if it has subject and object. Sometimes subject may not be written in passive voice but it does not mean that it has no subject. Such sentences have subject but the subject is so common or familiar or known that if even it is not written in passive voice, it gives full meaning.
For example
Cloth is sold in yards.

The following tenses can also not be changed into passive voice.
Present perfect continuous tense
Past perfect continuous tense
Future continuous tense
Future perfect continuous tense.