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1、一、if作为连词，引导条件状语从句，它表示的意思是“假如如果等。在复合句中如果主句用将来时，则 if引导的状语从句用一般现在时。例如：1. If ltdoesntram, we will go to the park next Sunday.如果天不下雨，卜周星期天我们将去公园2. If you ask him, he will help yo u .如果你求他，他将会帮助你。二、if还可以引导让步状语从句。这时if当作“即使是”，“虽说“解。例如：1. If shes poor, at least shes honest.虽说她很穷，但至少她还是诚实的。2. If I am wrong, y
2、ou are wrong, too.即使说我错了，那么你也不对。3. Ill do it, even if it takes me all the afternoon虽然会花费我一下午的时间我还是要做这事。三、if作为连词还可以引导宾语从句。引导宾语从句时和whether意思相同。例如：1 Lily asked if / whether she liked it.莉莉问她是否喜欢它。2. She asked if / whether they had a cotton one.她问是否他们有一件棉织的。四、if引导时间状语从句，当if做“当“或无论何时”解而不含有条件之义时，if从句中的时态与
3、主句 中 的时态相同。例如：1. If youmix yellow and blue you get green.你将黄色与蓝色混合，便会得到绿色2. If she wants the servant, she rings the bell.每当她需要仆人时，她便按铃。五、if后接否住动词，用于感叹句中，表示沮丧、惊奇等。例如：1 Well, if I havent left my false teeth at home? X倒霉我把假牙丢在家里了 ！2. And if he didn*t try to knock me down!(What do you tliuik he did?He t
4、ried knock me down?)六、用于虚拟语气中川从句中用过去式，表示不可能实现，大概不会实现或提出作为考虑的假左条件。 例 如：1. If you were a bird, you could fly 假使你是只鸟，你便会飞了。2. If I asked him(if I were to ask him) for a loan, would he agree?如果我向他借贷他会答应吗？七、if从句中用过去完成式，表示过去未实现的条件(例如由于不可能实现或某人之未能实行)。1.If they had startedearlier they would have arrivedin t
5、ime If they had not started when they did, they would not be here now.第三，在文学体栽中，if有时可省略，然后将主语与所用的限疋动词(尤苴是were, had, should)倒置。 例如：1. Should it (=if it should)be necessary, I will go.倘若有必要，我会去的。2. Were l(=if I were) in your place, I would do the same 3. Had l(=if I had)known earlier, I would come.如果我
6、早一点知道，我就会来。八、讦与其它一些介词的连用。(1) as与if连用，它的意思是：“仿佛、好像”同样引导从句。例如：1. It isnAt as if we are rich. (We are not rich.)我们不像富有的样子(即我们不富有)2. It isnt as if he doesnt know the ruler, (i - c. He does know the rulers -)(2) as if还可以引导感叹句。例如：As if I would allow it!(i. c. I would certainly not allow it!)(3) if与only连用。
7、常表示愿望或一个未实现的条件，尤英用于感叹句中。例如：1. If only he arrives in time他若能及时到达就好了 ！2. If only she would marry me!但愿她能嫁给我！o1 - if可引导状语从句一 “如果，假使.就现在，过去，未来可能实现之事加以推测时If you are tired, we will go straight home 如果你疲倦了，我们就直接回家If you have finished reading this book, please return it to me Do you mind if I open the windo
8、w? 如果我打开窗子，你介意吗？I shall tell him if he comes 如果他来了 我会告诉他2 用于虚拟语气中If you knew how I suffered, you would pity meIf I were you, I would help him 如果我是你，我会帮助他If I had known, I wouldnt have done it 假使我知道， 我就不会做那件事If he had followed my advice, he would nt be in such difficulty nowIf it should rain tomorrow
9、, I shall not shant, wont come3 if 可引导宾语从句，”是不是基本等同于 whetherAsk him if it is true 问他那是不是真的I wonder if he is in the school 我不知道他是否在学校IF 引导的条件状语从句，后半句即主句为将来时，从句则应为现在时。例女口 ： If he walks to school at 6:50am ,he will be late for schoolIF 有时候也引导虚拟语气例女1 : If I were you, I would go with him.since 用法：1. 用作介词
10、，意思是”从以来，自从“，常与完成时连用f 歹！ J 女 D : I have nt written home since Christmas.2. 用作副词，意思是“从那时以后(一直到现在)”， ever since 从那以后一直便加强调 since 的 用 法)。His first book was a great success and he has been famous as a writer ever since.3. 用作连词4. 引导原因状语从句，相当于nowthat，意思是“因为，既然，鉴于”。Since the rain has stopped, lets go for a
11、 walk.since 表示原因时，语气比 because 弱，表示的原因是对方已知的，或是句中不很重要的部分，是附带 的原 因，因此引导的从句不是信息的焦点，不能回答 why 的提问，也不能用在强调句型中作为被强调 的内容。 since 引导的从句往往放在主句之前。5. 引导时间状语从句，此用法较复杂。(1) 若 since 引导的状语从句的谓语动词是终止性动词(如 go,come,leave, start, begin 等) 的过去时，则从句表示的时间是“从该动作开始的那一时刻起”。这是最常见的一种用 法。如：He has studied very hard since he came t
12、o our school.(2) 右since引导的状语从句的谓语动词是持续性动词或表示状态的静态动词(如live, stay, study, learn, smoke, be等)的过去时，则从句表示的时间是”从该 动作或状态的完成或 结束时算起”。如：I haverft eaten snakes since I was a student at school.(3) 若since引导的状语从句的谓语为持续，性动词或静态动词的现在完成时，则表示动作和状态延续到现在(说话时刻)，其表示的时间的起点应从动作发生之时算起。如： He has written to me frequently sinc
13、e I have been ill.(4) It is + 段时间 + since. / It has been + 段时间 + since.表示”从起已有多长时间了”。两句型意思相同，前者是规范说法，后者是口语说法。用法：since从句中常用一般过去时，若从句中是持续性动词的一般过去时，则表示该动作结束有多 长时间了。如：It is three years since he smoked.if条件句不一般，几个要点记心间； 条件句，放在前，逗号要放句中间。条件句表可能，主句多用将来时： 条件句表事实，主句常用现在时。在if引导的条件状语从句中，如果从句谈论的是一个有可能发生的事实及英产生的相
14、尖的结果，主句用一 般将来时态，从句用一般现在时态。女口：We can walk there if we cant find a bus .If it rains tomorrow ,we will not go to the zoo. What will you do if you find a panda in dan ger.如果if条件句谈论的是重复发生和预示要发生的情景和事件，则主从句大多用一般现在时态。如：If bears are in dan ger ,they attack people.注意：在if引导的条件状语从句中，if和条件句位置灵活，可直接放在主句后而，若if条件句放
15、句首；从句后而要 加逗号，和主句隔开。还要注意前后时态一致原则as if的用法as if好像，好似She spoke to me as if she knew me.她和我说话的神情，好像她早就认识我似的。(=as though)一、as if从句的作用1、在look, seem等系动词后引导表语从句。She looks as if she were ten years younger.It seems as if our team is going to win 看来我们队要胜了。2、引导方式状语从句。She loves the boy as if she were his mother. The child talked to us as if he were a grown up.二、as if还可用于省略句中如果as if引导的从句是”主语+系动词”结构，可省略主语和系动词，这样as if后就只剩下名词、不定式、形容词(短语)、介词短语或分词。 ng)g.She left the room hurriedly as if (she was) angry.三、as if从句的语气及时态1. as if 从句用陈述语气的情况。当说话者认为句子所述的是真实的或
