IGBT直流斩波电路的设计

If的用法if 的条件从句分为真实条件从句和虚拟条件从句。真实条件从句：真实条件从句适用于主将从现，是假设有可能发生的。一 位置在if引导的条件状语从句中，既可以放在主句之前，也可以放在主句之后。若if条件句放句首，从句后面要加逗号，和主句隔开。You will catch the bus if you get up early. = If you get up early, you will catch the bus.二 时态1.（主祈从现）主句：一般现在时态（包含祈使句和含有情态动词can, may, must等的句子）从句：一般现在时态2. （主将从现）主句：一般将来时态从句：一般现在时态3.从句:现在进行时，主句:一般将来时 IfyouarelookingforPeteryoullfindhimupstairs.如果你是在找彼得，上楼就会找到他。4if从句用现在完成时，主句用一般将来时 IfyouhavefinisheddinnerIllaskthewaiterforthebill.如果你吃完了，我就叫服务生来算账5.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：Ifbearsareindanger,theyattackpeople.三 注意事项1. 在if 引导的条件状语从句中，当主句是一般将来时态时，不能使用be going to 结构，而要用will.2. 在if 引导的条件状语从句中,不能使用some,而要用any.四 同义句转化1. 借助于祈使句+andor+简单句转化，and表示顺承，or表示转折。例如：(1)If you work hard, youllpass the exam. Work hard, and youll pass the exam. (2)If you dont hurry up, youll be late. Hurry up, or youll be late.2. 借助于介词with或者without 来转换例如：If you help me, Ill finish my job soon. With your help , Ill finish my job soon.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。虚拟条件从句:虚拟条件从句是假设不太可能发生的。IF的虚拟语气有三种：内容 从句 主句与现在事实相反 动词过去式（be用were） would/could/should/might+动原与将来事实相反 should/were to+动原 would/could/should/might+动原与过去事实相反 had+过分 would/could/should/might+ have+过分下面三句依次是上面三种情况的例句：If I were you ,I would/could/should/might do that.If I had know that yesterday, I would/could/should/might have told you .If it should rain , the crops would/could/might be saved. If引导的宾语从句 一 if 引导宾语从句时，意思为：是否，从句的时态通常要和主句保持一致。 【例句】 I want to know if he lives there. 我想知道他是否住在那儿。 He asked me if I could help him. 他问我是否能帮他。【考查点】if、whether引导的宾语从句的区别 1. if或whether引导宾语从句时，作“是否”讲，常放在动词ask、see、say、know和find out等后面。一般情况下，两者常可换用，在口语中多用if。 【例句】 Nobody knows whether / if it will rain tomorrow. 没有人知道明天是否下雨。 Lucy asked whether / if they had a cotton sweater. 露西问他们是否有棉制的毛衣。 二. 注意事项：if或whether引导宾语从句时，要注意三个方面，即连词、语序和时态。1. if或whether既不能和that 或其他连词（副词）同时使用，也不能省略。 【例句】 我不知道他今天是否会来。 误 I dont know that if(whether) he will come here today. 正 I dont know if(whether) he will come here today. 他走过来看看猫是否出了毛病。 误 He came to see what if (whether) there was anything wrong with his cat . 正 He came to see if (whether) there was anything wrong with his cat . 2.if或whether引导宾语从句时，虽具有疑问的意义，但其从句应用陈述句的语序。 【例句】Did you know the way to the hospital? The old woman asked me. The old woman asked me if / whether I knew the way to the hospital. 老妇人问我是否知道去医院的路。 3. if或whether引导的宾语从句应和主句的时态保持一致。即主句为一般现在时，从句用任一种时态；主句为一般过去时，从句则用过去时中的任一种时态。 【例句】 I dont know if (whether) he has come here. 我不知道他是否来过这儿。 Tom asked if (whether) I had read the book. 汤姆问我是否看过这本书。 三If 和whether两者不能互换的情况1. 只能用whether的情况 在动词不定式之前只能用whether： 【例句】 I cant decide whether to stay. 我不能决定是否留下。 2. 在whether or not 的固定搭配中： 【例句】 I want to know whether its good news or not. 我想知道它是不是好消息。3. 在介词后，只能用whether： 【例句】 His father is worried about whether he loses his work. 他的父亲担心他是否会失去工作。 4.用if会引起歧义时： 【例句】 Could you tell me if you know the answer? 这句话有两种意思： “你能告诉我你是否知道答案吗？”或“如果你知道答案，请告诉我，好吗？”。 如用whether则可避免歧义。 【考题链接】 I dont know _ they could pass the exam or not. A. why B. if C. that D. whether 答案：D 解题思路：本题考查if与whether的区别，在whetheror not的固定搭配中通常只用whether。 练习题：句型转换 1. Do they want fried chicken? He asked the boys.(改为宾语从句) He asked the boys _they _fried chicken. 2. Was the watch made in Shanghai? I dont know.(改为宾语从句) I dont know _the watch _made in Shanghai. 3. “Do you want to try something new?” Toms mother asked him.(同义句转换) Toms mother asked him _to try something new. 答案：1. if / whether, wanted 2. if / whether, was 3. if / whether he wanted一、用所给词的适当形式填空 1.Ifyou_(feel)tired,you_(have)tohavearest. 3.Ifthere_(be)fewertrees,there_(be)morepollution.5.IfMarcia_(live)alone,she_(keep)apetparrot.6.Lana_(buy)anewdressiftheoldone_(be)outofstyle.9.Peter_(send)meabeautifulsouvenirifhe_(tour)Spain.10.IfMrGreen_(say)Iamhard-working,myparents_(feel)glad.11.I_(go)tothebeachifit_(notrain)thisweek.13.He_(write)alettertohisgrandparentsifhe_(get)hisreportcardthisweek.14.Ifshe_(get)uplate,she_(notcatch)theearlybus.15.Peter_(major)inEnglishifhe_(pass)theexamsinPekingUniversity.16.如果努力学习，你就会取得好成绩。 Ifyou_,you_good_.17.他如果看电视太久了，他的父母会不高兴。 Ifhe_TVtoomuch,hisparents_unhappy.18.如果我们为他组织生日聚会，大家都会来的。 Everyone_ifwe_abirthdayparty_him.19.如果明天不下雨，我会和你一起去公园的。 Ifit_tomorrow,I_gotothepark_you.21.如果他有空，他会帮助你的。 Ifhe_,he_you.23.你如果抄同学的作业就不能学好各门功课。 You_goodatallthesubjectsifyou_theotherstudentshomework. 单项选择（1.Ifyou_totheparty,youllhaveagreattime.A.willgo B.went C.go D.going2.Itwillbealongtime_Peter_hiswork.A.since,hasfinishedB.after,finishesC.when,willfinishD.before,finishes.3.Whatwillfather_usfromJapan?A.takeB.bringC.carryD.make4.Weifangisfamous_kites.A.forB.toC.onD.with5I_hertheanswerifshe_me. A.cantell,willaskB.willtell,willaskC.wouldtell,askD.willtell,asks6.Whatareyougoingtodotomorrow?-Wellgotothelibrarytomorrowifit_. A.isntrain B.rainC.wontrain D.doesntrain7.Doyouknowwhenhewillcomebacktomorrow?-Sorry,Idontknow.Whenhe_back,Illtellyou.A.comesB.willcomeC.comeD.maycome 8.Whatwillyoudoifyou_totheoldfolkshomevisit?A.goB.wentC. goingD.willgo9.IfIeat_food,Illbeveryfat. A.toomany B.manytooC.toomuch D.muchtoo10.Illgivethebooktohimifhe_herenextSunday.A.willcomeB.comesC.iscomingD.came11.There_anEnglishfilminourschooltomorrow. A.isgoingtohaveB.willhaveC.isgoingtobeD.has12.Couldyoutelluswhere_？A.willthenextOlympicGamesheldB.thenextOlympicGameswillbeheldC.wouldthenextOlympicGamesbehel