see,hear,feel,smell,listen to,notice,watch等感官动词.我们一般用现在分词来表示正在进行的未完成的动作；用不定式表示已经完成的动作,经常性的动作,或非延续性的动作,此时的动词不定式在主动语态中不带to,在被动语态中必须带to.例如：
I saw him playing basketball on the playground.我看见他在操场上打篮球.（正在打,
I saw him play basketball on the playground.我看见他在操场上打篮球.（打完球了,全过程；主动句,无to）
He was seen to play basketball on the playground.有人看见他在操场上打篮球.（被动句,要带to）
I noticed him buy a pen in the shop.我看见他在商店里买了一支笔.（buy为非延续性动词）
We often hear her sing in the classroom.我们经常听见她在教室里唱歌.（经常性的动作）