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）。 

Can you lift this heavy box?（体力） 

Mary can speak three languages.（知识）