1) They deliver the post twice a day
The post is delivered (by them) twice a day
2) You never use your computer
Your computer is never used (by you).
3) I am not going to the party. They have not invited me.
 I am not going to the party. I haven't been invited (by them)
4) They are examining the students in this class
The students are being examined (by them) in the class.