一：主动语态中，使役动词let， make， have后面跟的不带to的不定式作宾语补足语，变为被动语态，要把to还原上：
1、Let Mary have a rest.
(主动语态)
Mary is let to have a rest.
(被动语态)
2、The boss made the workers work twelve hours a day.
(主动语态)
The workers were made to work twelve hours a day by the boss.
(被动语态)
二：主动语态中，感官动词feel， listen to， hear， look at， see， watch， notice， observe， find等后面跟的不带to的不定式作宾语补足语，变为被动语态，要把to还原上。例句：
1、I saw Mary come into the room. (主动语态)
Mary was seen to come into the room by me.(被动语态)
2、I heard Tom sing.(主动语态)
Tom was heard to sing by me.(被动语态)
