在功能上介乎主动词和助动词之间的一类结构，称为半助动词。常见的半助动词有be about to, be due to, be going to, be likely to, be meant to, be obliged to, be supposed to, be willing to, have to, seem to, be unable to, be unwilling to等。它们可与主动词搭配构成复杂动词词组并表示情态意义，有时也可与助动词搭配。例如：
The boy has to make a living by begging.
男孩不得不通过乞讨维持生计。
He is unwilling to help me with my English.
他不愿帮我学习英语。
Every clause is obliged to obtain a finite verb.
每个句子必须包括一个限定动词。
We were about to start when it rained.
下雨时，我们即将出发。
He seems to be very disappointed.
他好像非常失望。
I don’t have to buy a new house.
我不必买新房子。
He didn’t seem to feel in love with her.
他好像没有爱上她。
半助动词的其他用法
带有半助动词作为动词词组的组成部分的句子，有的可以转换为“ It…that-分句”结构，而有的却不可以。从这个角度看，半助动词可分为两类：一类是可以作上述转换的，其中包括be cer-tain to，be（un）likely to，appear to，happen to，seem to等。例如：
They are certain to win the game．他们肯定会在比赛中获胜。
→It is certain that they will win the game．
He is likely to let you down．他可能使你失望。
→It is likely that he will let you down．
He appears to have many friends．看来他有许多朋友。
→It appears that he has many friends．
He seems to be enjoying himself．看来他玩得很快活。
→It seems that he is enjoying himself．
I happen to have had some nasty experiences in dealing with him．
我恰巧有过一些和他打交道的不愉快的经历。
→It happens that I have had some nasty experiences in deal－ing with him．
Some members of the committee seem to have been bribed．
委员会的一些成员看来是受了贿赂。
→It seems that some members of the committee have been bribed．
另一类半助动词不可以作上述转换，这一类包括be about to，be bound to，be going to，be to，had better，have to，have got to，tend to 等。例如：
He id adout to arrive．他即将到达。
He is bound to win．他一定会获胜。
The boy tended to be late．这孩子老是迟到。
不可以转换为：
*It is about that he will arrive．
*It bound that he will win．
*It tended that the boy was late．
半助动词也可用于there－存在句。例如：
There's going to be a storm．就要有暴风雨了。
There's to be an investigation．要作一次调查。
There has to be a mistake．一定有错。
There's sure to be some rain tonight．今晚一定有雨。
There's likely to be a large audience．观众人数可能很多。
There's certain to be trouble at the factory．厂里一定有麻烦。
There seems to be a widespread change of attitude
看来态度有普遍的改变。
There appears to be no doubt about it．
看来这是毫无疑问的了。
凡是能够作上述句型转换的半助动词，在there-存在句中，也可作类似的转换。现在，我们再从相反的方向来看这种转换关系：
It is likely that there'll be a large audience．
→There is likely to be a large audience．
It is certain that there is trouble at the factory．
→There is certain to be trouble at the factory．
It seems that there's a widespread change of attitude．
→There seeme to be a widespread change of attitude．
It appears that there's no doubt about it．
→There appers to be no doubt about it．
顺便说一句，如果“It…that＋there-存在句”的主句谓语动词是is said，is believed等被动态，那末转换过来的there-存在句的谓语动词也应是被动态。例如：
It is said that there's trouble at the factory．据说厂里有麻烦。
→There's said to be trouble at the factory．
不过，这里的is said to（be）已经不是半助动词了。
在结束本节以前，还要提一下半助动词的省略式问题。在口语中，had better，have got to和be going to有时可用省略式。例如：
You（'d） better go now．你最好现在就去。
You（'ve）just got to help．me．你得帮帮我呀。
Where（are） you going to get the money from？
你准备从哪儿去弄这笔钱？
be to结构在新闻标题中可以单用to-不定式表示：
UNESCO CHIEF TO VISIT AFRICA
联合国教科文组织领导人将出访非洲
要注意，下面一些结构中的to-不定式是作主语补语用，而不是半助动词be to+不定式。例如下面两句均作“最好的还在后头”（The best is still ahead of us，in the future）解释，其中的to come和to be均作主语补语用：
The best is still to come．
The best is yet to be．
还有一些结构在语法上可作不同理解，既可看作形容词词组作后置修饰语，也可看作关系分句的省略形式。例如：
Those likely to arrive after 7 p．m．should notify Mrs Thomas be－fore 14 March…
可能在下午7点以后到达的人需于三月十四日前通知托马斯夫人……
In desperation it（指her mind）lit upon the figure of her father al－ready broken in health and now about to die．
在绝望中，她脑海里突然出现了她那早已病魔缠身，如今已奄奄一息的父亲的身影。
Becarse I was the player least likely to be used in that game，I was the one sent to forage．
由于我是一个最不可能被选拔参加那场比赛的队员，因此就派我出去搜索粮秣。
在上述诸例中，likely to arrive after 7 p．m．，now about to die，least likely to be used等都既可看成形容词词组，也可看作关系分句的省略形式。如果作后一种分析，就涉及半助动词的问题．例如Thoselikey to arrive after 7 p．m．相当于Those who are likely to arrive．．，在这里，are likely to是半助动词（本书是这样处理的）；当然，who are likely to arrive…也可以看作是“主—动—补”结构。这也就是说，像“are likely to+不定式”这一结构既可作为一个整体来看待（即看作半助动词+不定式），也可分开来看待（即将该形容词看作主语补语）。由于likely，sure，certain等都是形容词，从而也可以接受副词的修饰（如上述后两例），并有比较等级形式。例如：
It might be very well if dramatists were philosophers，but in point of fact they are as little likely to be so as are kings．
要是剧作家同时又是思想家的话，那该有多好，可是，事实上剧作家不可能同时又是思想家，正如国王毫不可能同时又是思想家一样。
在这里，as little likely to be so as are kings似以看作形容词词组作主语补语为宜，否则便把事情搞得过于复杂化了。
