Question

This letter was posted last night change into voice

Answer 1

last night this letter posted.

In an english there are different ways of expressing your voice. It can be either active or passive. One more method is direct and indirect voice.

But here we will discuss the active and passive method.

So letter was posted last night is a oassive sentence which is converted into active above.

Answer 2

Last night that letter was being posted by someone.