英语if语句的用法

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!