英语：情态动词被动语态

1. 如果感官动词后的是V-ing作为宾语补足语,变被动时不需要加to.
如果感官动词后的是V-pp作为宾语补足语,变被动时也不需要加to.
只有当感官动词后的是不带to的不定式（即所谓的动词原形）作为宾语补足语,变被动时才需要加to.
2. 情态动词变被动语态结构特征：情态动词+ be + 动词的过去分词
如：Young trees must be watered often.
Your mistakes should be corrected right now.
The door may be locked inside.
Your homework can be handed in tomorrow.
像can (could), may (might), must, need, ought to, dare (dared), shall (should), will (would) .都是