(1)情态动词can只有两种时态形式,现在式can和过去式could,而be able to有多种时态形式。
如:Mary can play the piano. She has been able to play it since she was 5. 玛丽会弹钢琴。她五岁起就会弹了。
(2)用在过去时中,could经常表示能够做某事,事实上不一定去做,而was∕were able to则表示“过去做成了某事”。在否定句中两者可通用。
如:He could swim across the English Channel. But he didn′t feel like it that day.
他能游过英吉利海峡,但那天他不想游。
Yesterday I was able to get home before the heavy rain. 昨天我在下大雨前赶到了家里。
3、must和have to:
must表示主观意志,而have to表示由于客观因素不得不做某事。must没有过去式,除在间接引语中可用于表示过去时间,在直接引语中表示过去时间应该用had to代替。
如:I told her that she must give up smoking. 我叫她必须戒烟。
We had to get everything ready that night. 我们那晚得把一切准备就绪。
4、would和used to:
(1)usedto表示过去与现在或过去某时与后来的情况有不同,而would只表过去的情况。
如:People used to think that the earth was flat. 过去人们认为地球是平的。(现在人们不这么认为。)
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