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It+be+时间段+since/before…句型句型意义备注Itwill/won’tbe+时间段+before…要等多长时间才……/过不了多长时间就……Itwas/wasn’t+时间段+before…过了多长时间才……/没过多长时间就……Itis/hasbeen+时间段+since自从……以来已经有多长时间了1.从句中用短暂性动词表示动作持续了多久2.从句中用延续性动词表示动作结束了多久3.从句要用一般过去时Itwillbetwoyearsbeforehecomesbackfromabroad.要等两年他才回国。Itwon’tbetwoyearsbeforehecomesbackfromabroad.过不了两年他就回国。Itwastwoyearsbeforehecamebackfromabroad.过了两年他才回国。Itwasn’ttwoyearsbeforehecamebackfromabroad.没到两年他就回国了。Itis/hasbeentwoyearssincehecamebackfromabroad.他回国已经两年了。Itis/hasbeentwoyearssincehelivedabroad.他不在国外已两年了。Howlongdoyouthinkitis_________hearrivedhere?--Nomorethanhalfayear,Ibelieve.Itwastwoyearslater_______hecamebackfromabroad.Itistwohours______hespendsonEnglisheveryday.Itwasat6o’clock_______Igotuptoday.Itwas6o’clock_______Igotuptoday.1.Itis/was(high/about)time+(that)…“是做……的时候了”，从句的谓语动词用：①过去式（be用were）②should+动词原形，should不可省,表示虚拟语气。Itistimethathecamebackfromabroad.他早该回国了。2.It+is+thefirst/second…time+that…ItisthefirsttimethatIhavevisitedtheGreatWall.3.It+was+thefirst/second…time+that…ItwasthefirsttimethatIhadvisitedtheGreatWall.4.It+be+时间点+when…引导的时间状语从句ItwillbetomorrowmorningwhenweleaveforBeijing.我们去北京时将是明天早晨。Itwas9o’clockwhenwegothome.我们到家时已经九点了。