1. Two leopards were seen by us last summer.
2. An essay is being written by someone at the moment.
3. A dictionary was used by them in language lessons.
4. She was given ten roses by him yesterday.
5. Was the mice caught by you?
6. Dinner was being prepared by them at 5 pm last Monday.
7. How are English sentences formed by you?
8. Pictures weren't taken by us last holidays.
9. Ten kilometers are going to be run by her next weekend.
10. A pineapple is being eaten by me.
11. Jeans are going to be worn by him at the party.
12. A new house has been built by them.
13. Can the window be opened by you?
Please note that I very well may be wrong in some of these answers because I am just using common speech that I think sounds correct (there may be specific grammar or verb rules I am unfamiliar with). Also, please note that in some cases, the object is not necessary. For example, number 13 can also be "Can the window be opened?" I do not know which version you are looking for.