"They were surprised by a knock at the door." is passive voice.
The active voice sentence "Who is knocking at the door?" can be changed to passive voice as "By whom is the door being knocked?" In the passive voice sentence, the subject of the active voice sentence ("who") becomes the object of the preposition "by," and the verb "knocking" is changed to "being knocked," with the helping verb "is" moved to the appropriate position.
This
To change a sentence into passive the sentence must be active. 'Shut the door please' is not an active sentence, it is an imperative sentence, it has no subject. You could write something like this - You are asked to shut the door - but this is a bit awkward.
I think Knock on the door makes more sense. Knock at the door.... What do you knock on at the door? the wall?
No, knock is a noun (a knock on the door) and a verb (to knock on the door).
By whom was the door opened?
This sentence is a command, it has no subject and it cannot be changed to a passive sentence.
let the door be shut
The active voice sentence "Who is knocking at the door?" can be changed to passive voice as "By whom is the door being knocked?" In the passive voice sentence, the subject of the active voice sentence ("who") becomes the object of the preposition "by," and the verb "knocking" is changed to "being knocked," with the helping verb "is" moved to the appropriate position.
This
The door was being knocked at by some body.
To change a sentence into passive the sentence must be active. 'Shut the door please' is not an active sentence, it is an imperative sentence, it has no subject. You could write something like this - You are asked to shut the door - but this is a bit awkward.
I think Knock on the door makes more sense. Knock at the door.... What do you knock on at the door? the wall?
No, knock is a noun (a knock on the door) and a verb (to knock on the door).
here i am i stand at the door and knock if any one here my voice and opens the door i will come eat with them and thy with me
it depends what knock on the door?
I started to knock on the door. I will knock before I open the door.