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　　Wehadcleaned(clean)theroombeforeyoucame.[beforeyoucame(在你来之前),表明动作在过去（came）以前,用过去完成时.]

　　It'stoolate.Ihavetogo(go)immediately.[haveto+动词原形,表示“不得不”]

　　Whenwearrived(arrive)thestation,thetrainhadleft(leave).[arrived(到达)是过去的动作,而火车到达发生在“arrived”之前,用过去完成时：hadleft]

　　MustIfinish(finish)myhomeworkfirst?[情态动词must后面必须接动词原形]

　　ThesearesomephotographsthatItook(take)lastmonth.[时间是“lastmonth(上个月)”,过去时间,用一般过去时.]

　　Bytheendoflastweek,Ihadread(read)threebooks.[“bytheendof+过去时间(到……末之前)”,表示“过去以前”,动词用过去完成时.]

　　Thegirlwhowesaw(see)justnowismyclassmate.[时间是“justnow(刚才)”过去时间,用一般过去时.]

　　Bobdidn'tgo(go)toschoolyesterday,becausehewasill.[时间是“yesterday(昨天)”,用一般过去时.又根据意思理解,因为“生病”所以应该“没去上学”,用否定式.]