不明白半助动词是什么 它有什么样的用法

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.
他好象非常失望。
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.
他好象没有爱上她。