第一大類：虛擬條件句

條件句有真實條件句和虛擬條件句兩種。真實條件句所表示的假設是有可能發生的，此時主句不用虛擬語氣；而虛擬條件句則表示一種假想，與事實相反或不大可能會發生，此時用虛擬語氣。
如：If I have time, I will go.
假若我有時間，我就去。 （陳述語氣）
If I were you, I would go.
假若我是你，我就去。 （虛擬語氣 ）
第一講復習
與現在事實相反：從句：If 主語+過去時（Be動詞用were）主句：主語+should/would/could/might+do
與過去事實相反：從句：If 主語+had+done；主句：主語+should/would/could/might+have done
對將來的主觀推測：從句：①if+主語+were to do 主句：①主語+should/would/could/might+do；從句：②if+主語+did/were 主句：②主語+should/would/could/might+do；從句：③if+主語+should+do 主句：③主語+should/would/could/might+do

第二講 虛擬語氣在條件句中的用法全套解析

第一類：穿越，即錯綜條件句

有時，虛擬條件句中，結果主句和條件從句的謂語動作若不是同時發生時，虛擬語氣的形式應作相應的調整。這種條件句叫錯綜條件句。
① 從句的動作與過去事實相反，而主句的動作與現在或現在正在發生的事實不符。
If I had worked hard at school, I would be an engineer, too.
如果我在學校學習刻苦的話，我現在也會成為一個工程師了。
If they had informed us, we would not come here now.
如果他們通知過我們的話，我們現在就不會來這里了。
If it had rained last night, the ground would be wet now.
要是昨晚下過雨的話，現在地面就會是濕的。
You would be much better now if you had taken my advice.
假若你當時聽我的話，你現在就會好多了。
②從句的動作與現在事實相反，而主句的動作與過去事實不符。
If he were free today, we would have sent him to Beijing.
如果他今天有空的話，我們會已經派他去北京了。
If he knew her,he would have greeted her.
要是他認識她的話，他肯定會去問候她了。

第二類：隱身，即省略條件句

1. 省略主句或從句：有時，虛擬條件句中，主、從句可以省略其中一個，來表示說話人的一種強烈的感情。
①省略從句
He would have finished it.
他本該完成了。（主句可能是：If he had started it earlier 如果他早些開始）
You could have passed this exam.
你本能通過這次考試的。(主句可能是：If you had studied harder 如果你再努力一點）
②省略主句
If I were at home now.
要是我現在在家里該多好啊。
If only I had got it.
要是只有我得到它了該多好啊。
2. 省略 if虛擬條件：有時可將條件從句的連詞if省略，但此時應用倒裝句型，即把從句中的were, should, had 等提到句首。例句：
Were I Tom, I would refuse.
如果我是湯姆，我會拒絕。（標準原句：If I were Tom, I would refuse.）
Should it be necessary, I would go.
假若有必要，我會去的。（標準原句：If it were necessary, I would go.）
Had it not been for the bad weather we would have arrived on time.
若不是天氣壞，我們就準時到達了。（標準原句：If it had not been for the bad weather we would have arrived on time.）
3. 省略含蓄條件句：在特定的上下文或一目了然的情況下，if條件句可以省略，或使用介詞短語、副詞或非謂語動詞來代替if條件句。這種虛擬語氣表達形式叫做含蓄條件句
①條件暗含在短語中
We didn't know his telephone number; otherwise we would have telephoned him.
我們不知道他的電話號碼，否則我們就會給他打電話。（暗含條件是副詞otherwise）
Without your help, we wouldn't have achieved so much.
沒有你的幫助，我們不可能取得這么大的成績。（暗含條件是介詞短語without your help）
But for your help, I would not have succeeded in the experiment.
如果沒有你的幫助，我的實驗就不會成功。（暗含條件是but for your help）
②條件暗含在上下文中
I would not have done it that way.
我是不會那么做的。（可能暗含if I were you）
I was busy that day. Otherwise I would have come to help you.
我那天很忙。否則我會來幫你的。（可能暗含if I hadn't been so busy.）
You might come to join us in the discussion.
你可以參加我們的討論。（可能暗含if you wanted to）
I would have bought the DVD player.
我是會買下那臺影碟機的。（可能暗含if I had the money）
But for the storm, we should have arrived earlier.
要不是碰到暴雨，我們還會早些到。（可能暗含if it had not been for the storm）