1) The Colosseum was completed by the Romans in 82 AD.
2) The key was lost yesterday (можно ещё добавить в конце by me, хотя вовсе необязательно)
3) The text will be translated tomorrow (опять же, можно добавить в конце by us, а можно и не добавлять).
4) When he stops speaking, many questions will be asked (по желанию в конце by me)
5) All the meaning of the world were explained by the teacher.
6) London is visited by millions of tourists every year.