1. These facts are memorized by someone.
2. A beautiful city was situated on a river. - Тут причастие уже в страдательном залоге.
3. A lot of skyscrapers in New York was built by the people.
3. Many skyscrapers in New York were built by the people.
4. A little polar bear lived in a city zoo. - Имхо, перевести в страдательный нельзя.
5. Some endangered species were saved by the people.
6. Daisies and pansies are watered by my grandma in her garden.
7. Last spring Dick was on a tour through Australia. - Имхо, перевести в страдательный нельзя.
8. A small island in the middle of a lake was discovered by them.