If I am free, I will come to see you.
= I will come to see you if I am free.
If it snows tomorrow, we will go skiing.
If it stops raining, we can go out.
If you heat the ice, it turns into water.
条件从句(if):主语+had done 主句might/would/should/could+have done
if+ 主语+动词过去式(be用were)主句 might/would/should/could+do
if+主语+动词过去式(be用were)或主语+should do或主语+were to do
Tom got to the station in time because he started earlier.
If Tom had started late, he would have儿童癫痫病治疗 missed the train.
Do you think the thief entered through the door?
No, if he had, I don't believe, he would have broken the living room window.
If the book weren't so expensive, I would buy it.
If you didn't live so far away, we would be able to visit you more.
What would you do if you lost your passport in a foreign country?
Why hasn't he come? If he should not come on time, we would have to put off the trip.
If she hadn't work hard at English in the past, she wouldn't work as well as a secretary in a large company now. 混合时间的虚拟语气 从句为对过去的虚拟，主句是现在。
Had we not used an out-of-date timetable, we wouldn't have missed the train.
if引导从句的倒装从句相当于 if we had not used an out-of-date timetable, 同样，were还有should 在从句中都可以提前，省略if.
If only 要是...就好了
If only I were younger!
If only I had worked harder in my teenage years!
If only I should travel tomorrow!
句型1：If +主语+过去时，主语+ should (could, would, 或might) +动词原形
(1) I石家庄治疗癫痫病的医院f I were you, I would study hard.
(2) If it rained, I would not be here now.
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.
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.
(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.天津什么医院看羊羔疯好
(11) Should it rain tomorrow, we would stay at home.
(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。
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乌鲁木齐癫痫哪家好 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
(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.