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 。一、 can, could 1) 表示能力（体力、知识、技能）。 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只有一般现在式和一般过去式；而be able to则有更多的时态。 如：Ill not be able to come this afternoon. 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 the
