1 條件句中的虛擬語氣：從句中提出一種與客觀現實不相符或根本不可能存在的條件，主句會產生的一種不可能獲得的結果。條件句中的虛擬語氣根據不同的時間有三種不同的形式。
(1) 將來時的條件句中的虛擬語氣（should + 動詞原形或were to + 動詞原形）。如：
If he should go to Qing Hua University, he would make full use of his time. 如果他要上清華大學的話，他就會充分利用他的時間了。
If he were to come here, he would tell us about it. 如果他要來的話，他會通知我們一聲。
(2) 現在時的條件句中的虛擬語氣。（動詞過去式(be 用 were)或would / should / might / could + 動詞原形）如：
If he were free, he would help us. 要是他有空的話，它會幫助我們的。
If he studied at this school, he would know you well. 如果他在這所學校學習的話，它會對你很熟悉。
(3) 過去時的條件句中的虛擬語氣。（had +動詞過去分詞或would / should / might / could have + 動詞過去分詞）如：
If I had seen the film, I would have told you about it. 我如果看過這場電影，我會把電影內容告訴你了。
If I had got there earlier, I would have met Mr. Li. 如果我早點到那兒，我就會會到了李先生。
2 運用條件句中的虛擬語氣時，須注意的幾個問題
(1) 當從句的主語為第三人稱單數時，謂語動詞若是系動詞be時，可用 was 代替 were。但在倒裝虛擬結構及 if I were you, as it were 中，只能用 were。如：
Were I ten years younger, I would study abroad. 要是我還年輕十歲的話，我會去國外學習。
If I were you, I would try my best to grasp the chance. 要是我是你的話，我要盡力抓住這次機會。
(2)
有時，虛擬條件句中，主、從句的動作若不是同時發生時，須區別對待。
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。如：
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，我現在也會使工程師了
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裡了。
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。如：
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。
③從句的動作與過去發生的情況相反，而主句的動作與現在正在發生的情況相反。如：
If it had not been raining too much, the crops would be growing much better. 如果天不下太多的雨的話，莊家會長得更好。
If he had been working hard, he would be working in the office now. 要是他工作一直努力的話，他現在已進了辦公室了。
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