Question

when did they deliver the mail? change into passive voice

Answer 1

There are two voices in english language.

One is called active voice and the other one is called the passive voice.

The passive voice focusses more on the person or object that undergoes the action rather than the person or the object performing the action. e.g. the sentence could be changed in to passive voice as, when the mail was delivered by them.  

Answer 2

Hey mate !!!

Answer :-

When the mail was delivered by them ?

Hope this helps !