有什么关于if的英语语法知识?

有什么关于if的英语语法知识?

if是英语中出现频率很高的连词,并且它的用法也很多.学生感到很难掌握,为了让同学们更多更好地了解其用法及其语法功能.现结合教学中遇到的情况,予以归纳,以供参考.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 itdoesn’train,we will go to the park next Sunday.如果天不下雨,下周星期天我们将去公园.2.If you ask him,he will help you．如果你求他,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 she’s poor,at least she’s honest． 虽说她很穷,但至少她还是诚实的.2.If I am wrong,you are wrong,too． 即使说我错了,那么你也不对.3.I’ll 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后接否定动词,用于感叹句中,表示沮丧、惊奇等.例如：I.Well,if I haven’t left my false teeth at home!真倒霉,我把假牙丢在家里了!2.And if he didn’t try to knock me down!(What do you think he did!He tried knock me down!)你猜他想做什么?他想把我撞倒!六、用于虚拟语气中,if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定条件.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?另外,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).例如：1.If they had startedearlier,they would have arrivedin time．要是他们早些动身,他们便可及时到达了.2.If they had not started when they did,they would not be here now．如果他们那时不动身,现在他们就不会在此地了.第三,在文学体栽中,if有时可省略,然后将主语与所用的限定动词(尤其是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 I(=if I were) in your place,I would do the same ． 如果我处于你的位置,我也会这样干的.3.Had I(=if I had)known earlier,I would come．如果我早一点知道,我就会来.七、if与其它一些介词的连用.首先,as与if连用,它的意思是：“仿佛、好像”同样引导从句.例如：1.It isn’t 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 isn’t as if he doesn’t know the ruler．(i．e．He does know the rulers．)他不像不懂规则的样子.(即他懂得规则) 另外as if还可以引导感叹句.例如：As if I would allow it!(i．e．I would certainly not allow it!)好像我会答应似的!(我才不会答应哩!) 其次,if与only连用.常表示愿望或一个未实现的条件,尤其用于感叹句中.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!3.If only she had known about it!（But she didn’t know.） 她那时要是知道（但她不知道）这事就好了!详见：http://www.ryedu.net/syy/zxyy/200812/10896.html