make let 等词变被动语态时怎么还原to能具体将一下吗

(1)在主动语态中,make ,let ,hear ,see ,watch ,notice 等词,其后跟省略to的动词不定式(动词原形),在变为被动语态时,必须还原动词不定式符号to.如:
We heard him sing this song in the room.
He was heard to sing this song in the room.
The boss made him work 14 hours a day.
He was made to work 14 hours a day.
I see him walk to school
He is seen to walk to school.这个要靠大脑记啊!