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call [误] Ill call at Mr Brown. [正] Ill call on Mr Brown. [误] Ill call on Mr Browns home. [正] Ill call at Mr Browns home. [析] 作拜访讲时，at后面接访问地点，而on后面接访问的人。 call on drop in visit call on比较正式的为公务的访问，如：We were called on by the old students. 而drop in则是比较随便的走走，顺便拜访，如：If youre free, drop in. 而visit则是更正式的外交访问或友好往来，如：My schools headmaster will visit America next week. can [误] A blind man can not judge colours. [正] A blind man cannot judge colours. [误] I cannt call for you at ten. [正] I cant call for you at ten. [析] can的否定形式应为cannot或cant. [误] Its only six oclock. That mustnt be the postman. [正] Its only six oclock. That cant be the postman. [析] must用来表示一种肯定的推断，如：She must have some problems. She keeps crying. 但在否定句中则要用cant， 要表示对过去的推测则要用must+have+过去分词的表达法，如：The lights have gone out.A fuse must have blown.而对过去的否定推测则多用cant+have+过去分词，如：I dont think he can have heard you. Call again. | 
