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.