1. Mathematics IS very difficult.
2. Our football team are winners. (на самом деле, тут можно так и оставить, потому что "team" - это collective noun и может употребляться с is or are в зависимости от контекста. Почитайте про это в интернете и выберите, что Вам ближе)
3. There are many monkEYs in the tree.
4. The dishEs are on the table.
5. My TEETH are yellow.
6. How many womEN are there in your company?
7. AreN'T my shirtS washed yet/Haven't my shirts been washed yet?
8. Not many citIEs ARE clean and safe.
9. There ARE many problemS in this exercise/THESE EXERCISES.
10. There are six librarIEs in our school.
Заранее прошу прощения, если что-то осталось незамеченным.