学习好资料欢迎下载主要适用于下面的几种情况:1)表示截止现在业已完成的动作By now, I have collected all the data that I need . 到现在为止, 我已收集到了我所需的全部资料。She has read 150 pages today . 她今天已看了 150 页。We haven't met for many years . 我们已多年没见了。They have developed a new product . 他们研制成功了一种新产品。2)表示发生在过去而对现在产生影响、带来结果的动作Have you had your dinner? 你吃晚饭了吗?She has been to the United States. 她已去美国了。You have grown much taller. 你长高了许多。3)表示过去发生的动作持续到现在,并可能还要延续It has been five years since he joined the army . 他参军五年了。They have learned English for eight years . 他们已学了八年的英语了。So far we've only discussed the first five chapters. 至今我们还只讨论了前五章。几点注意1)一个句子应该用什么时态只能取决于它需要表达的意思,以及它所处的语言环境。例如:He speaks English . (一般现在时,说明动作发生的经常性。)He spoke English when he was in New Zealand . (一般过去时,说明动作发生的时间。)He is speaking English. (现在进行时,说明动作正在进行。)He has spoken English for three years since he came to the USA. (现在完成时,这里说明动作的总和。)He has been speaking English since he came to the USA. (现在完成进行时,强调动作的连续性。 )2)在含有时间状语从句或条件状语从句的主从复合句中,如果主句的谓语动词是一般将来时,那么从句的谓语动词就只能用一般现在时来表示将来时;如果主句的谓语动词是过去将来时,那么从句的谓语动词就只能用一般过去时来表示过去将来时。学习好资料欢迎下载例如: I will not play tennis if it rains tomorrow. I would not play tennis if it rained the next day. 3) 有些动词表示无法持续的动作,它们一般不宜用于进行时态中。这类动词常见的有:be , believe , consist , find , forget , hate , have , hope , hear , know , like ,love , notice , prefer , remember , see , seem , smell , suggest ,...