被动语态构成：情态动词 + be + 动词过去分词
The patient can be saved by him.
A fire could be caused by the faulty operation.
She may be surprised by you.
The file must be checked by you.
被动语态构成：情态动词 + have been + 动词过去分词
A fire could have been caused by the faulty operation.
She may have been surprised by you.
The file must have been checked by you.
She should have been told by them.