请把下列句子变为被动语态：我只知道被动语态的构成是：be＋动词过去分词1.The teacher is keeping the pupils at school for a revision.2.You must finish the article before Friday.3.They promised Mary a new doll for her birthday.

1. The pupils are being kept by the teacher at school for a revision.
现在进行时的被动语态：S.+be(is/am/are)+being+done
2. The article must be finished by you before Friday.
带有情态动词的被动语态：S.+情态动词+be done
3. Mary was promised a new doll by them for her birthday .
一般过去时的被动语态：S.+be(was/were)+done

The pupils are being kept by the teacher for a revision.
The article must be finished before Friday.
Mary was promised a new doll for her birthday by them.

1.The pupils at school are kept for a revision by the teacher.2.The article must be finished by you before Friday.3.Mary was promised a new doll for her birthday by them.