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1. 表示过去的动作对现在的影响或结果，常用recently, lately, ever, never, once, before, yet, just等作时间状语。 He hasn't heard any news from his son lately. 他最近没有得到他儿子的任何信息。 Have you ever been to London? 你曾去过伦敦呢？ 2. 表示开始于过去某一时刻，一直持续到现在而且还有可能继续下去的动作。常用so farup till now, since, for a long time, in the past/last few years, these days等作时间状语。 So far no life has been found outside the earth.迄今为止，没有在地球外的星球上发现生命。 He has learned 5，000 English words since he went to college.自从他上大学以来已经学会了5,000英语单词。 “This/It is＋the ＋序数词＋time ＋that...”句式中，that后的从句用现在完成时。 This is the second time that I have visited the university. 这是我第二次参观这所大学。 非延续性动词的否定形式可以与表示延续时间的状语连用即动作不发生的状态是可以持续的。 I haven''t received his letter for almost a month.since与for用法比较 since 用来说明动作起始时间 for 用来说明动作延续时间长度 I have lived here for more than twenty years. 我在这里住了二十多年了。 | 
