Question

after they had shut the doors nobody could get in change into passsive​

Answer 1

Explanation:

I completely agree that no sane person would ever utter those words. But that said, the passive voice of the sentence ‘shut the door’ is indeed ‘let the door be shut’.

‘Shut the door’; ‘let someone shut the door’- Active

‘Let the door be shut’ (by someone)- Passive

Directives with let :

First person imperatives can be formed by preposing the verb let’. The same applies to third person subjects. (And the same applies to passives :) These are elevated in tone.

‘Let us all work hard’ is not a suggestion; but a first person imperative; a command. ( ‘Let's party’, however, is a suggestion.)

‘Let no one think that a Prime Minister's life is easy.’ Imperative.

Except for the ‘let me’ type, these are rather archaic in form and uncommon in Modern English.

This type of imperative, in which let is no more than an introductory particle, should be kept separate from the ordinary second person imperative of let as a transitive verb.

Answer 2

Answer:

after they had shut the doors