1. You must hand in your report before the end of this month

=> Your report had to hand before the end of this month.

2. I had just finished the job when the factory closed

=> The job had been just finished when the factory closed.

3. No one has opened that box for the past hundred years

=> That box has not been opened for the past hundred years

4. Are you going to mail these letters soon?

=> Are these letters going to be mailed soon ?

5. He should have finished the report yesterday

=> The report yesterday should have been finished yesterday.
6.  Scientists began to carry out researches on SIDA two years ago.

-> Researches on SIDA began to be carried out by scientists two years ago.