专题五情态动词和虚拟语气一、几组情态动词的区别1 . can 和 be able tocan 与 be able to 都可以表示能力,但两者在用法上有点差异: can(could) 表示主观能力,不表示意愿; be able to 表示主观意愿,强调要克服困难去做某事。它的将来时用 will be able to 。例如:My grandma is over seventy , but she can still read without glasses.He is able to give up his bad habits.2 . must 和 have tomust 和 have to 都可以表示“必须”,但有几点区别:(1)must 强调“内在的职责”、“义务”, 而 have to 强调“外界压力”、“不得已而为之”。(2)have to 可用于多种时态, 而 must 一般用于现在时,其 过去时与将来时分别由 had to 与 shall/will have to 代替。(3) 在回答 must 引起的问题时,如果是否定的答复,不能 用 mustn't , 而要用 needn't 或 don't have to , 因为mustn't 是“一定不要”、“一定不能”、“禁止”的意思。例如:1)You must come to the classroom before eight.2)It is raining hard outside , but I have to leave now.3)“Must we do it now ?”“ No , you needn't.”3 . used to + do , be used to + doing 和 be used to + do(1)used to + do 意为“过去常常做某事”,“过去一直做某事”;be used to + v. ing/n. 意为“习惯于”; be used to + do 意为“被用来做某事”。(2)used to 只表示过去, 而 be used to + v. ing/n. 可表示现在、过去或将来。例如:1)He used to smoke.Now he doesn't.2)He's quite used to hard work/working hard.3)The knife is used to cut bread.4 . can't 与 mustn'tcan't 表示“不能”,是指无能力; mustn't 表示“不能”,指的是“禁止,不允许”。在口语里, can't 也可表示“禁止,不允许”, 但语气不如 mustn't 强。例如:He is so young that he can't go to school.There's a meeting inside.You mustn't make any noise.5 . can 和 may 考试中主要测试 can , may 或 could , might ,表示可能 性的区别及对 may 构成的疑问句的回答。(1)can , could , may , might 都可以表示可能性。 can , coul...