摘要：大学英语四级语法要从词类、时态语态、从句等方面进行学习。以下是精品学习网为考生提供的2013年大学英语四级考试语法的考核重点分析和复习方法指导。
??"在……条件下发生了或将发生……事"，这是人们在语言交流过程中常见的句式。英语中通常用if引导的从句表达这一概念，但决不限于if条件句。英语中的条件状语有多种表现形式，体现了英语语言的丰富多彩。
??1?用介词短语表达
??能用来表示条件的介词主要有with，without，but for等。注意：如果with， without短语所陈述的情况完全有可能成为事实，动词用陈述语气;如果所陈述的情况与事实相反，则用虚拟语气。含but for的句子一般用虚拟语气。但介词短语本身常常并不表示条件，而是隐含了条件。例如：
??Without revolutionary theory there can be no revolutionary movement.没有革命理论，就不可能有革命运动。(陈述语气)
??Without air，the earth would undergo extreme changes in temperature.没有空气，地球就会出现极冷极热的变化。(虚拟语气)
??With a little care you could have avoided this accident.如果小心一点，你本可以避免这次事故。
??But for my brother's help，I would not have finished the work.如果不是我兄弟帮忙，我就不会完成这项工作。
??Such floods would have caused thousands of deaths in the past.(in the past隐含了if thefloods had occurred in the past)这样的洪水在过去会造成数千人的死亡。
??2?用短语连接词引出
??能引起条件句的短语有：on condition that(条件是)，as long as(只要)，in case(假使)，but that(要不是)等。其中含but that从句的主句谓语动词须用虚拟语气。例如：
??I will lend you this book on condition that you return it to me before Friday.如果你能在周五以前还给我，我就借你这本书。
??He would have said no but that he was afraid.要不是害怕，他就会说"不"了。
??This project can go on as long as it is not raining.只要不下雨，工程就可以继续进行。
??3?用分词引出
??英语中，被用来表示条件的词还有providing(that)(假如)，provided(that)(假如)， supposing(that)(假定)，given(假如)。这组词都是由动词演变而来，除given外，都已被认定为连词。例如：Supposing it rains，what shall we do?如果下起雨来，我们怎么办?
??Provided /Providing(that)there is no op- position，we shall hold the meeting here.如果没有不同意见，我们将在这里举行会议。
??Given good health，I hope to finish the
work this year.假若健康状况良好，我希望今年完成这项工作。
??4?隐含在上下文中
??有些假设条件没有明示，需根据上下文加以判断。例如：
??He can't be ill or he wouldn't have come.他不可能生病了，否则他不会来的。(隐含了...if he were ill，...)
??She had intellect，otherwise I should have scorned her.她聪明，否则我就会看不起她。(隐含了...if she had not had intellect，...)
??He would have given you more help，but he was very busy.他本来要给你更多的帮助，只是他太忙了。(隐含了...if he had not been busy，...)
??注意：含有or，otherwise的句子，谓语动词并不都是虚拟语气，要根据所表达情况的真实与否而定。如下面的句子就不必用虚拟语气。
??Wear your coat or you'll be cold.穿上外套，要不然你会冷的。
??
??Hand in your application now，otherwise you will miss the opportunity.现在就把申请交上来，否则的话你就会失去机会的。
??Exercise：Fill in the blanks with the proper words.
??1. ____ he saw it，he would not have believed it.(But for /But that /Suppose)
??2.Motors will run well ____ they aremaintained regularly.(unless /otherwise /provided)
??3.But for your help，I ____?.(had failed /should have failed)
??4.I am getting old-otherwise I____ her.(will keep /would keep /have kept)
??5.He will do the work ____?you pay him.(providing /unless)
??6.The old woman would have been drowned but that a passer-by ____?her out of the water.(had dragged /would drag / dragged)
??7.We ____?better under more favorable conditions.(could have done /had done / did)
??8. ____ he arrives before I get back， please ask him to wait.(In case /Given)
??9.He would have come to see you，but he____ very busy at the time.(were /was / is)
??10.A less brave man ____ to work in the enemy headquarters.(did not dare /wouldn't have dared /dared not)
Key：1.But that 2.provided 3.should have failed 4.would keep
??5.providing 6.dragged 7.could have done 8.In case 9.was ??10.wouldn't have dared