被动语态和情态动词结合
(1) 被动语态也可以和情态动词结合
can/could + be + 动词过去分词
may/might
must/have to
should/ought to
had better/would rather
used to/would
特殊：need + doing / to be + 动词过去分词
注意：need doing 等价于 need to be done。都表示被动语态，不过need doing常用。
被动语态和情态动词结合表示：什么应该被怎么样，必须被怎么样，加上情态。
1. The food could be taken away. 食物可以被带走
2. The food might be taken away. 食物可以被带走
3. The food must be taken away. 食物必须被带走
4. The food should be taken away. 食物应该被带走
5. The food needs taking away. 食物需要被带走。这里的need情态动词作实意动词,有人称时态的变化，needs根据前面不可数名词变单三形式。need+doing表被动
6. The food had better be taken away. 食物最好被带走。
7. Books used to be returned in two days. 书过去常常在两天内被还回去。
上面的除了第5个句子，其他都是情态动词+动词的被动语态构成be+done形式。
练习
1. The room may (可能) be cleaned.
2. The door must (必须) be locked.
3. The house should (应该) be furnished.
4. The tree had better (最好) be watered now.