掌握英语时态的使用是提高语言表达准确性和流畅性的关键。现在完成时是英语中一个非常重要的时态，它不仅能够表达过去发生的动作对现在造成的影响，还能描述从过去某一时刻开始一直延续到现在的动作或状态。此外，过去完成时则常用于表示在过去某个时间点之前已经完成的动作或状态。
<例句>
I won't believe you until I've seen you in the room with my own eyes.
我要亲眼见到你在这个屋子里才相信你。
<语法分析>
在时间和条件状语从句中，可用现在完成时态代替将来完成时。延续性动词可用于现在完成时，可接表示段的时间状语。非延续性动词也可用于现在完成时，但不接表示段的时间状语。有时在口语中，可用一般过去时代替现在完成时。有些动词，如hope, plan, mean, expect和think等有时可用过去完成时来表示一个本来打算做而未做，曾经设想而未实现的事情。在时间和条件状语从句中，过去完成时可代替过去将来完成时。
<触类旁通>
(1) He has got constant help from her since he has known her.
自从他认识她以来，他一直得到她的帮助。
语法分析：为了强调从句谓语动词从过去某一时刻开始，一直延续到现在，可用现在完成时态。
(2) I have never heard of her since she left.
自她离开后湖，我从未听说过她。
语法分析：since从句如果用现在完成时，其谓语动词一定是延续性的，如果是非延续性的，则只能用一般过去时。
(3) The vistors have stayed here for three days.
来访者在这儿住了三天。
语法分析：延续性动词可用于现在完成时，可接表示段的时间状语。
(4) The young man has joined the Party.
这年轻人入了党。
语法分析：非延续性动词也可用于现在完成时，但不接表示段的时间状语。
(5) I had intended to come over to see you, but was prevented from doing so.
我本打算来看望你，但受阻没有来成。
语法分析：用过去完成时来表示一个本来打算做而未做，曾经设想而未实现的事情。
(6) We would get into trouble as soon as the water had run out.
水一用完，我们就会陷入困境。
语法分析：在时间和条件状语从句中，过去完成时可代替过去将来完成时。
<巩固练习>
(1) When you _____[learn] computer, you'll find it a bridge to so much knowledge.
(2) The film _____[put] on at this cinema for three days.
(3) I _____[meet] him twice this morning.
(4) I _____[hear] of you before.
(5) This is the most wonderful film I ____[see}.
(6) In the last few years there _____[be] great changes in computers.
(7) She must _____[be] here last week.
(8) He should _____[let] us through the forest yesterday evening.
(9) The old man _____[live] here since 1980.
<参考答案>
(1) have learned (2) has been put (3) have met (4) have heard (5) have ever seen (6) have been (7) have seen (8) have let (9) has lived