(1) 情态动词及助动词之后一般要省略to,如:
He can speak several languages. 他能说好几种语言。
We must work hard. 我们必须努力工作。
注意:在used, ought, have, be之后的不定式要带to。如:
He used to come here at eight. 他过去常在八点来这里。
(2) 在感官动词feel, hear, look at, listen to, notice, observe, sense, see, watch及使让动词make后做宾语补足语的不定式前的to须省略。如:
I heard them break a glass in the next door. 我听见他们在隔壁打破了一个玻璃杯子。
注意:当这些动词,除let, have没有被动语态外,变为被动语态时,不定式前的to不能省略。如:
They were heard to break a glass in the next door. 听见他们在隔壁打破了一个玻璃杯子。
(3) 不定式作介词besides, but, except的宾语或系动词be的表语时,而在介词besides, but, except或系动词be前有do的某种形式,不定式前的to须省略。如:
We can do nothing but wait there. 我们除了在那儿等待之外,没有其它的办法。
(4) 不定式在cannot but, had better / best, rather than, sooner than, would rather, would sooner, know better than等短语之后时,不定式前的to常被省略。如:
I would rather watch a TV play than play cards. 我宁愿看一场电视剧,也不愿打牌。
(5) 用why向第二人称you 提问,而you被省略时,动词前不能用to,从而构成Why (not) do…? 如:
You look tired. Why not have a good rest. 你看起来很疲倦,何不好好休息一下呢?
(6) 在go hang, hear say, hear tell, let go, let fly, make believe, make do 等固定搭配中,不定式前的to常被省略。如:
I used to hear say that he came from Canada. 我过去常听说他是加拿大人。