感官与使役动词的被动还原 to
主动句里省掉的 to，变成被动句后必须还回来。
see / hear / watch / make / let + sb + do → sb + be + 过去分词 + to do
要点
- 主动：感官动词（see / hear / watch / notice / feel）和使役动词（make / let / have）后接不带 to 的不定式：I saw him cross the street.
- 变被动后 to 必须还原：He was seen to cross the street.（不能说 He was seen cross…）
- make 的被动：be made to do —— The child was made to work all day.
- let 一般不用被动，改用 be allowed to do：Mother let me go. → I was allowed to go.
- help 后的 to 可有可无，被动后习惯保留：He was helped to finish the work.
- 还原 to 只管"看见他做过"那一格：如果主动句看到的是正在进行（I saw him crossing），变被动仍保留 -ing：He was seen crossing the street. —— 不转成 to cross。
- make 一进被动就带出"被迫"的口气：They made us wait → We were made to wait —— 主动的"让"在这一头得另措辞，中文也听得出来。
例句
The boy was made to copy the text five times.
那个男孩被迫把课文抄了五遍。
He was seen to enter the building at nine.
有人看见他九点进了那栋楼。
We were made to wait for more than an hour.
我们被迫等了一个多小时。
The star was heard to sing in the next room.
有人听到那位明星在隔壁房间唱歌。
He was seen crossing the street at midnight.
有人看见他半夜正在过马路。
The workers were made to work all night.
工人们被迫干了一整夜。
易错提示
- 口诀：主动省 to，被动还 to——see / hear / watch / make / let 五兄弟都守这条规矩。
- but / except 前面有 do 时后面不定式可省 to：He did nothing but wait.（与之相对， couldn't choose but to…）
- 省 to 与句型无关：疑问句里照样省 —— Did you see him fall?（不说 see him to fall）；判 to 只由"感官 / 使役 + 宾语 + 动词"这一格决定。