名人教英文：infinitive何时不跟to？（2）！
上周提到有些动词配 bare infinitive，即宾语后面不加 to，直接跟动词原型，如 let/ help/ make/have someone do something。这几个动词属于 causative verbs（使役动词）。另一类配 bare infinitive的动词属于 verbs of perception（感知动词），例如see、watch、observe、notice、hear、feel和smell︰
?mr chan saw his domestic helper (家务助理) sweep the floor.（而不是mr chan saw his domestic helper to sweep the floor.）
?she watches her son play basketball every saturday.
?i heard the alarm clock go off an hour ago.
?i turned around when i felt someone tap (轻拍) me on the shoulder.
?he noticed a stranger enter the building.
感知动词也可以配现在分词（即「动词 + ing」），例如：
?i saw my friend walking down the street.