半情态动词，是指与can, could, may, might, must等这类纯情态动词相对而言的。
根据《张道真英语语法》，半情态动词主要包括两类：一类是指像need和dare这类既可用作情态动词又可用作实义动词的动词。如：You need a new bike. 你需要一辆新自行车。(need为实义动词)You needn’t clean the windows. 你不必擦窗户。(need为情态动词)另一类是指像 be able to, have to, used to, had better (best), would rather这类词组或短语，它们在意义和语气上与情态动词用法比较相似，所以也被归入到情态动词的用法中。如：He was able to speak five languages. 他能讲5种语言。We have to go to a meeting now. 我们现在得去开一个会。We’d better not tell him. 我们最好还是不告诉他。I think we had best sell it. 我想我们还是把它卖掉的好。The children’ d rather go by air. 孩子们更愿意坐飞机去。这一类就是语法书上说的“半情态动词”。
