虛擬條件句往往指不能實現的或純假想的情況，可以對過去現在將來進行假想。
1）表示與現在情況相反，if從句用一般過去時態，be動詞用were,主句謂語用「would/should/could/might+動詞原形」。如
If I had a mini-computer on hand, I would certainly let you use it.
如果我現在手頭有一臺微電腦，我肯定會借給你。
If I were you, I would try my best to grasp the chance.
如果我是你，我會努力抓住這個機會。
Were I ten years younger, I would study abroad.
要是我年輕十歲，我就去國外學習。
If you tried again, you might succeed.
如果你再試一次，你一定會成功。
2）表示與過去事實相反或對過去的假想。If從句用過去完成時（had +過去分詞），主語謂語用「would/should/could/might+have+過去分詞」。如
If the policeman had arrived earlier, he would have seen the accident.
如果這個警察早些到達， 他就能看見事故了。
If you had come yesterday, you would have met George.
要是你昨天來，你就會碰到喬治了。
3）表示將來難以實現的假設推測或純假想的情況，從句謂語用一般過去式（be動詞用were),或「were to +動詞原形」，或「should+動詞原形」；主句謂語用「would/should/could/might+動詞原形」。如
If it should rain tomorrow, we should have to put off the picnic.
如果明天下雨，我們只好推遲野餐。
If it snowed tomorrow,our plan would be postponed.
萬一明天下雪，我們的計劃就得推遲。
If we should come tomorrow, we could meet the general manager.
如果我們明天真的能來，就能見到總經理。
If I were to do the job, I would do it in a different way.
要是我來做這項工作，我會用另一種方法。
4）有時候，主句和從句的動作不是同時發生，要區別對待：
⑴從句的動作與過去事實相反，而主句的動作與現在動作或狀態相反或不符。
If I had worked hard at school, I would be an engineer, too.
要是我再學校努力學習的話，我現在也會是工程師了。
If they had informed us, we would not come here now.
如果他們通知我們的話，我們現在就不會來這裡了。
⑵從句的動作與現在事實相反，而主句的動作與過去動作或狀態相反或不符。
If he were free today, we would have sent him to Beijing.
如果他今天有空的話，我們早送他去北京了。
If he know her, he would have greeted her.
要是他認識她的話，他肯定去問候他了。
5）當虛擬條件句的 謂語動詞含有were, should, had時，if可省略，而將were, should, had等詞放置於句首。
Should he agree to go there, we would send him there.
要是他答應去，我們就派他去。
Were she here, she would agree with us.
如果她再這兒，她會同意我們的。
Had he learnt about computers, we would have hired him to work here.
如果他懂得一些電腦知識，我們能就會聘用他來這裡工作了。
6）有時，主句或從句可以省略一個。如
If only I had got it. 要是我得到它該多好啊！
If I were at home now. 要是我現在在家裡多好！
He would have finished it.他本該完成的！
（省略if從句）
You could have passed this exam.
你本來能通考及格的。
（省略if從句）