请把下列句子变为被动语态：
问题描述：
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.
答
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.