注意： 感官动词和一些使役动词的不定式的主动形式要省去to，但在变成被动语态后需要加上to。能这样用的动词有:make, have, let, see, hear, notice, listen to, watch等。如： We often see him act like that. = He is often seen to act like that. 我们常常看到他这样做。