一、使役动词与to的省略
当不定式用于let, make, have等使役动词后作宾语补足语,不定式必须省略to.如：
Let me have another cup of tea. 给我再来一杯茶.
She had him dig away the snow. 她让他把雪挖走.
They made him tell them everything. 他们强迫他把一切全告诉他们.
但是,当使役动词用于被动语态时,其后的不定式则必须要带to.如：
他被迫一天工作20小时.
误：He was made work twenty hours a day.
正：He was made to work twenty hours a day.
注意,表示使役意义的let和have很少用于被动语态.
另外注意,force, oblige等虽然也表示“使”,但它们后用作宾语补足语的不定式必须带to.如：
They forced her to sign the paper. 他们强迫她在文件上签字.
The law obliged parents to send their children to school. 法律要求父母送子女上学.
二、感觉动词与to的省略
当不定式用于表示感觉的动词feel, hear, notice, observe, see, watch, look at, listen to等作宾语补足语时,不定式必须省略to.如：
We all felt the house shake. 我们都感觉这房子在震动.
I heard him go down the stairs. 我听见他下楼了.
Did you notice her leave the house? 她离开屋子你注意到了吗?
I watched her get into the car. 我看着她上了车.
但是,当feel后用作宾语补足语的不定式为to be时,则不能省略to.如：
They all felt the plan to be unwise. 他们都认为这个计划不明智.
注意,当这些动词变为被动语态时,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.
另外,若用作宾语补足语的不定式为完成式,则通常应带 to.如：
I noticed her to have come early. 我注意到她来得很早.
三、why (not) 与to的省略
在why (not)…?之后的不定式不能带to.如：
Why argue with him? He’ll never change his mind. 为什么要跟他争论? 他永远不会改变自己的看法.
Why not try going by boat for a change? 为什么不试着坐船去,这样也可以换换花样?
Why not wait till the winter sales to buy a new coat? 为什么不等到冬季大贱卖时再买一件新外衣呢?
四、介词except / but与to的省略
当不定式用作介词except或except的宾语时,该不定式有时带 to,有时不带 to,情形比较复杂,大致原则是：其前有do,不定式不带 to；其前没有 do,不定式通常带 to.如：
He likes nothing except to watch TV. 除了看电视外,他什么都不喜欢.
It had no effect except to make him angry. 除惹他生气外,没产生任何效果.
There’s little we can do except wait. 除了等待我们没有什么办法.
I could do nothing except agree. 我除了同意,没有别的办法.
五、help与to的省略
当动词help后跟一个不定式用作宾语或宾语补足语时,不定式可以不带to,也可以不带to.如：
Can you help (to) carry this table upstairs? 你能帮忙把桌子搬到楼上去吗?
I have lost my watch. Will you help me (to) look for it? 我把表丢了,你能不能帮我找一下?
但是,当help用于被动语态时,其后不定式必须带to.如：
Millie was helped to overcome her fear of flying. 米利被帮助克服了她的飞行恐惧.
另外,当不定式为否定式时,其中的to通常不宜省略.如：
How can I help my children not to worry about their exams? 我怎样才能帮助我的孩子们不为他们的考试着急呢?
六、know 与 to 的省略
不定式用于动词 know 之后作宾语补足语主要见于以下两类句型：
一是用于“know+宾语+to be”：当 know 为现在式时,其中的 to 不能省略；当 know 为过去式或完成式时,其中的to可以省略也可以保留.如：
I know him to be a fool.=I know (that) he is a fool. 我知道他是个傻瓜.
I’ve never known her (to) be late before. 我以前从不知道她迟到过.
二是用于“know+宾语+to do”：当 know 为现在式时,其中的 to 不能省略；当 know 为过去式或完成式时,其中的 to 可以省略也可以保留.如：
They know him to have been a spy. 他们知道他过去当过间谍.
I never knew him (to) do such a thing. 我从未见过他干那种事.
I’ve never known it (to) snow in July before. 我以前从未听说过七月份会下雪.
注意,在被动语态中,不定式必须带 to.如：
He has never been known to tell a lie. 从未有人听说过他说谎.
另外,当其中的不定式为完成式时,to也不可省略.如：
They knew the man to have been a thief. 他们发现此人曾是个小偷.
I have never known her to have failed. 我从来没发现她失败过.
七、bid 与 to 的省略
当bid表示“吩咐”时,用于其后作宾语补足语的不定式可以带to也可以不带to.如：
She bade me (to) come in. 她叫我进来.
He bade me (to) do the work. 他吩咐我去做那项工作.
但是,在当bid为被动语态时,其后的不定式必须带to.如：
I was bidden to do the work. 我被嘱咐去做那项工作.
八、不定式作表语与 to 的省略
当不定式用作表语时,其中的 to 原则上是不能省略的.如：
His aim is to do two years’ work in one. 他的目标是一年干两年的工作.
This wall is to keep people out of the garden. 这堵墙为的是不让人到花园里来.
但有一种例外,那就是当主语部分有动词do的某种形式时,用作表语的不定式可以省略to.如：
What you first do is (to) mix the egg with flour. 你先得把鸡蛋和面粉和好.
All I did was (to) touch the window, and it broke. 我只不过碰了一下窗户,玻璃就碎了.
九、并列不定式与to的省略
当两个或多个作用相同的不定式并列时,通常只需在第一个不定式前用 to,其余不定式前的 to 可以省略.如：
I’d like to lie down and (to) go to sleep. 我想躺下睡觉了.
Will you go to see the film or (to) dance? 你要去看电影还是要去跳舞?
I’ll go to see him myself and (to) ask him about it. 我将亲自去看望他并问一问那件事情.
但是,如果两者有对比关系,则后面不定式前的to不可省略.如：
To try and fail is better than not to try at all. 尝试而失败总比不尝试好.
十、固定搭配与to的省略
1. let类固定搭配：let fall (无意中说出),let fly(发射、攻击),let go (放开),let slip (放走),live and let live (自己活也让别人活)等.如：
She let slip a chance to work abroad. 她错过了出国工作的机会.
The dog’s got a stick between his teeth and he won’t let go. 那狗叼着一根棍儿不松口.
He aimed carefully and then let fly. 他仔细瞄准之后便开了枪.
I guessed what was happening from a few words she let fall. 我从她说的几句话中猜出发生什么事了.
2. make类搭配：make believe (假装),make do (凑合着用)等.如：
The boys made believe (that) they were astronauts. 男孩子们假扮成航天员.
We were in a hurry so we had to make do with a quick snack. 我们时间很紧,只好胡乱吃了顿小吃.