三、could 和 was /were able to
两者都用来表示能力和学到的技能:
He could / was able to drive a car when he was ten years old. 他十岁就会驾车。
After ten months' study he could / was able to read newspaper in English. 经过十个月的学习，他就能够读英文报纸了 .
但是说成功地做成一件事，通常不用could，而用was/ were able to, succeed in等: Finally we were able to reach the destination before dark. /Finally we succeeded in reaching the destination. 我们终于在天黑前到达了目的地。
四、must
must 的否定有多种形式。当must表示肯定的推测时,其否定通常用cannot/can't:
“She must be thirty by now.” “她现在必定有三十岁了”
“She can't be thirty. Her mother is only forty.” “她不可能有三十岁,她的母亲才四十岁。”
“She must have learned English for years before she went abroad.”
“She cannot have learned English for years. She had been linving in a remote village by then.”
“她出国前一定学过多年英语。” “她不可能学过多年英语,那时候她生活在一个遥远的村子里。”
但也有人(特别是在美国英语里)用must not / mustn't 表达同样的意思。以下两例摘自C. E. Eckersley and J. M. Eckersley 的A Comprehensive English Grammar （P.203）
Mr. GREen mustn't have received my letter， otherwise he would have replied before now. 格林先生不可能收到我的信,不然的话,他早就给我回信了。
I can hear the Browns' phone bell ring but no one is answering it; they mustn't be at home. 我听见布朗家电话铃响,但没人接,他们一定不在家。
当must作“必要、有义务”讲时,其否定是need not/ needn't:
You needn't hand in the report but they must. 你不必交报告,但他们必须交。
“Must we be there ten minutes earlier”
“No， you needn't .Please be there on time. ”
“我们必须提前十分种到达那儿吗?” “不必。请准时到那儿。”
mustn't / must not 表示“禁止”:
You mustn't smoke in the classroom. 不许在教室里抽烟。
