情态动词
情态动词有 can (could), may (might), must, have to, shall (should, will (would), dare (dared),
need (needed), ought to 等。 情态动词无人称和数的变化;不能单独使用,必须与其后的动
词原形构成谓语。
1. can, could
1) 表示能力(体力、知识、技能)。 例如:Can you lift this heavy box?(体力) Mary can
speak three languages.(知识) Can you skate?(技能) 此时可用 be able to 代替。Can 只有
一般现在时和一般过去式;而 be able to 则有更多的时态。
例如: I’ll not be able to come this afternoon. (一般将来时)当表示“经过努力才得以 做成
功某事”时应用 be able to,不能用 Can。如: He was able to go to the party yesterday evening
in spite of ...


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