Помогите с инглиш! Нужно преобразовать в страдательный залог предложения!

The letter was typed an hour ago.
Our exercises were corrected.
All the freshmen were greeted at the lecture.
The letter wasn`t posted yesterday.
Such things weren`t forgotten. 
A message for me will be left by them.
Ann won`t be told about it. 
By whom was America discovered?
By whom was this concert given?
When was Moscow University founded by Lomonosov?
What questions will be discussed at tomorrow`s meeting?
The work can be finished in three days.
These books must be returned to the library.