1.Three hours is a long time to wait.
2.Something is wrong.
3.Someboby pays extra money. 时态有点怪,但不错
4.Neither her sisters nor Mary is going to the school.
5.3000 Kms is far from here.
6.It was he discovered electricity. that/who不能省
7.Everybody is expected to do his duty.
1, 5的单复数应该可以,把时间,距离当整体。