模态辅助动词用法.doc

can / could 表示请求和允许（permission），could语气更委婉。 Can / Could I use your dictionary? Yes, I can. (回答时通常用can, 不用could) “You ____ borrow my notes provided you take care of them,” I told my friend. A. could B. should C. must D. can (口语多用) 表示能力（ability） She can swim, but she couldn’t last year. (can表现在或将来，could表过去) can和be able to的异同 表示“能力”，两者可互换 语法上，can属于情态助动词（modal auxiliary verb），它只有现在和过去两种时态形式，而be able to属于半助动词（semi- auxiliary verb），它有较多的时态形式，而且还可与其它情态助动词连用。 You might be able to persuade her. 你也许能说服她。 在针对过去情况时，两者的区别意义明显，不可混用。could do表示过去的一般能力，而was/were able to 表示做某件具体事情的能力，相当于managed to 或 succeeded in. We could save his life. 我们有办法救活他。 （但未必实施） We were able to save his life. 我们得以救活他。?（付诸实施） The fire spread through the hotel very quickly but everyone was able to (≠could) get out. 火势蔓延速度非常快，但是每个人都得以逃出酒店。 表示可能性（possibility），常用于否定句、疑问句或惊叹句中。 He can’t know my dress because I’ve moved it to another place. (不可能) What can he be doing all this time? 他会是一直在干什么呢？ If you’re not careful, you could get into even worse trouble. (可能会陷入更大的麻烦) ----The door was open. ----It ____ open. I had locked it myself and the key was in my pocket. A. can’t be B. mustn’t C. can’t have been D. mustn’t have been （“must + have + 过去分词”表示对过去发生的事情或状态进行推测,语气比较坚定，通常只用于肯定句; “can / could + have + 过去分词”表示对过去某种情况的推测，一般用于否定句和疑问句。） He ____ the 8:20 bus because he didn’t leave home till 8:25. A. couldn’t have caught (否定推测) B. ought to have caught (本应该) C. shouldn’t have caught (本不该) D. must not have caught may / might 表示允许（permission） --- May I ask you where you were born? --- Yes, you may. / No, you mustn’t / may not. 可以。/ 不可以。 表示可能性（possibility），常用于肯定句（疑问句和否定句常用can）. It may / might be true. (现在) He may / might get there in time, but I’m not sure. (将来) Linda may / might have missed the train. (过去) I don’t know whether he is there. He may / might not be there. He ____ unwisely, but he was at least trying to do something helpful. A. may have acted (肯定程度弱) B. must have