情态动词(modal verbs)
一.概念和特征
表示说话人对某一动作或是状态的态度的动词。情态动词具有一定的词义,但不能独立作谓语,要和后面的动词原形一起作谓语,它没有人称和数的变化,具有助动词的作用,即用来构成否定句、疑问句及用于简单回答。
二.类型
1. 只用作情态动词的:must can∕could may∕might ought to
2. 既可作情态动词也可作实义动词的: need dare
3.可作情态动词也可作助动词的: shall ∕ should will ∕ would
4.具有情态动词特征的动词短语:have to used to would rather had better
三.用法
1. can∕could
⑴. 表示能力,相当于be able to。
can you speak english?
he could swim when he was 5
can 只能用于一般现在时和一般过去式,而be able to可用于各种时态。
she has been able to play the piano for 3 years.
⑵.表示征求意见,许可,could比can的语气要委婉,相当于may 。
can i smoke here? yes,you can.∕no,you can’t.
she asked whether she could smoke here.
you can go with us if you like.
⑶. 表示推测,can比could可能性大, 用于肯定句、否定句、疑问句均可。
accidents can happen on such rainy days.
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