if的条件状语从句的用法
１.if引导的条件状语从句，在if引导条件状语从句中要用现在时态替代将来时，类似还有连词before，as soon as，while，when等。
２.零条件句/绝对真实条件句(Zero Conditional)
零条件句，又叫绝对真实条件句，用于阐述事实。用来表达在该条件下，结果一定会如此(如规则，客观规律、真理等)。结构是：If 主语 do/does，主语 do/does。其从句和主句的谓语动词通常为一般现在时。
３.第一条件句/相对真实条件句(First Conditional)
第一条件句又叫相对真实条件句，谈论将来可能发生的事情，并考虑这件事情的结果。强调偶然性或一次性的条件，也可以与绝对真实条件句同义。if从句用一般现在时或其他现在时，表示将来可能发生的事情(条件);主句用一般将来时，表示这件事情的结果。结构是：If 主语 do/does，主语 will do。
４.第二条件句/现在虚拟条件句(Second Conditional)
第二条件句是虚拟条件句的一种，即现在虚拟条件句(Unreal Present)，用来表达在现在或将来时间下不太可能或不可能发生的条件，这种条件句还可以用来表示试探性的、委婉的语气(可以理解为不大有自信，所以不太真实的条件)。结构是：①与将来事实相反的非真实条件句是If 主语 should do/were to do/did，主语 would/should/could/might do;②与现在事实相反的非真实条件句是If 主语 did，主语 would/should/could/might do。
５.第三条件句/过去虚拟条件句(Third Conditional)
第三条件句是表示与过去事实相反的虚拟语气，是用来表达如果(If)当时那样发生，另外一方面(当时)结果会如何。通常是指过去的事情，带有一切已经太迟而不能够补救的意思。结构是：If 主语 had done，主语 would/should/could/ might have done。
if句型的用法总结英语用法
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，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 she’s poor，at least she’s honest. 虽说她很穷，但至少她还是诚实的。
2. 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。
3. If I am wrong，you are wrong， too. 即使说我错了，那么你也不对。
三、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。
四、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。
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*。
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!
if从句用一般现在时，主句用一般将来时。
。 if从句用一般现在时，主句用may/might/can
。 if从句用一般现在时，主句用must/should
if从句用一般现在时，主句用一般现在时。
if从句用现在进行时，主句用一般将来时。
if从句用现在完成时，主句用一般将来时。
一、虚拟条件句：
条件状语从句是非真实情况，在这种情况下要用虚拟语气。
1.条件从句与现在事实不一致，其句型为：
句型1：If 主语 过去时，主语 should (could, would, 或might) 动词原形
(1) If I were you, I would study hard.
(2) If it rained, I would not be here now.
2.条件从句与过去事实不一致，其句型为：
句型2：If 主语 had 过去分词，主语 should(could, would, 或might) have 过去分词
(3) If the doctor had come last night, the boy would have been saved.
(4) If I had not studied hard, I would have failed in the exam last term.
3.条件从句与将来事实不一致，其句型为：
句型3：If 主语 过去时/ should 动词原形/ were to 动词原形，主语 should (could, would, 或might) 动词原形
(5) If it should rain tomorrow, we would stay at home.
(6) If I were to go to the moon one day, I could see it with my own eyes.
(7) If you missed the film tonight, you would feel sorry.
注意问题：
1.If条件句中绝对不可以出现would。
2.根据句中的时间状语，有时可能出现“混合虚拟”的情况，即主句可能是现在的情况，条件句也许是发生在过去的情况，但都要遵守上述句型。如：
(8) If you had studied hard before, you would be a college student now and you would graduate from a college in four years’ time.
3.条件句中如果出现were, had, should可省去if，将主语与这些词倒装。例如：
(9) Had the doctor come last night, the boy would have been saved.
(10) Were I to go to the moon one day, I would see it with my own eyes.
(11) Should it rain tomorrow, we would stay at home.
二、名词性虚拟语气：
在表示命令、建议、要求、惊叹时的名词性从句中需用虚拟语气，基本句型：
句型4：主语 (should) 动词原形
(12) Mother insisted that John (should) go to bed at 9 o’clock.(宾语从句)
(13) It was required that the crops (should) be harvested at once.(主语从句)
(14) It is surprising that she shouldn’t pass the math exam. (主语从句)
(15) The suggestion that he (should) be invited was rejected.(同位语从句)
(16) Their demand is that their wages (should) be increased.(表语从句)
注意：在这种句子中绝不可以出现would, must, could等。
三、虚拟语气在一些特殊词中的使用或含蓄条件句：
句型5：wish后的宾语从句
与现在愿望不一致：主语 过去时;
与过去愿望不一致：主语 had 过去分词 / could have 过去分词;
与未来愿望不一致：主语 would / could 动词原形。如;
(17) I wish I were you.
(18) I wish I had visited the White House when I was in the States.
(19) I wish I could meet you tomorrow at the party.
句型6：It’s time句型(当It’s time后用that从句时应该为“主语 should 动词原形”或“主语 过去时”)
(20) It’s time that you went / should go to school.
句型7：If only引起的感叹句相当于“How I wish 宾语从句”
(21) If only he could come! 他要能来就好了。
(22) If only I had known the answer! 我要早知答案就好了。
句型8：would rather, as if(though)引导的句子也需使用虚拟，表示过去的情况用过去完成时，表示现在与将来的情况
用过去时。
(23) I’d rather you posted the letter right away.
(24) I’d rather you had returned the book yesterday.
(25) She loves the children as if they were hers.
(26) Alan talked about Rome as if he had been there.
句型9：without, but, but for, but that, otherwise引起的短语或句子常暗含着虚拟条件。
(27) Without you, I would never know him.
(28) But for your cooperation, we wouldn’t have done the work so well.
(29) But that she was afraid, she would have said no.
(30) I would be most glad to help you, but I’ am busy now.
(31) I should have come to the party yesterday, but I was busy.
(32) I am busy now; otherwise I would do you the favor!
句型10：If it were not for ... / If it hadn’t been for ... (要不是因为……)，其后面的主句也需使用虚拟。
(33) If it were not for his help, I wouldn’t go home now.
(34) If it hadn’t been for the determined captain, all the passengers on the board wouldn’t have been saved.
