我們知道由if 引導的條件句有兩種形式，一種是真實的條件句，也就說是某事很可能發生，條件是可能存在的，主句中某種情況發生的概率也是非常高的。例如If you intend using humor in your talk to make people smile, you must know how to identify shared experiences and problems.(2002。Text1 )(如果你想在談話中用幽默來使人發笑，你就必須知道如何識別共同的經歷和共同的問題。)
另一種就非真實條件句，通過虛擬語氣來表達。例如I would have arrived much earlier if I had not been caught in the traffic.(要不是交通堵塞，我本會來得早一些。)在這句話中，假設“不堵塞”，但是真實情況并不是這樣，所以用虛擬語氣。一般有6種常見的形式：
1)與現在事實相反的非真實條件句，從句中用一般過去時，主句中用would/could/ should 加動詞原形，be 動詞的過去式為were.
If you then examined the European national youth teams that feed the World Cup and professional ranks, you would find this strange phenomenon to be ever more pronounced.(2007,text 1 )(查看為世界杯和專業隊輸送人才的歐洲國家青年隊，你會發現這一奇怪的現象更加明顯。)
2)與過去事實相反的非真實條件句，從句用過去完成時，主句用would/could/should +have done .
If he had played last season,however,he would have been one of 42. (2008 text3 )(如果他打了上個賽季，他將只是42個(最高的人)中的一個)
3)與將來事實相反的非真實條件句，從句用should do 或 were to do 結構，主句中用 would /could/should 加動詞原形。
If you were to examine the birth certificates of every soccer player in 2006’s World Cup tournament, you would most likely find a noteworthy quirk: elite soccer players are more likely to have been born in the earlier months of the year than in the later months.(2007 text 1)(如果查看2006年世界杯聯賽每個足球運動員的出生證，你很可能發笑一個值得注意的奇怪現象：好的足球運動員都在一年的頭幾個月而不是后幾個月出生。)
4)混合時態的虛擬。指的是主句和從句分別表示兩個時態，動詞形式依照上述三種形式進行相應的變化。
If he had put all the important files in the safe yesterday, his boss wouldn’t be scolding him so angrily now.(如果昨天他把所有重要文件都放在保險箱中，他的老板現在也不會這個嚴厲的說他。)
5)省if將從句中的系動詞或助動詞提前到主語之前，變成倒裝句，考生對這種形式應該理解和掌握。
Had it not for the timely investment from the general public, our company would not be so thriving as it is.(1998 )(如果不是群眾投資，我們的公司不會像現在這么興旺。)
6)用介詞或連詞表示虛擬.比如with,without, under…condition, but for.
And if one received a grape without having to provide her token in exchange at all, the other either tossed her own token at the researcher or out of the chamber, or refused to accept the slice of cucumber.(2005 text 1 )(如果一只猴子甚至沒有用石塊就能得到葡萄，另一只猴子就會向研究人員扔石頭，或者把石頭扔出房間，或者拒絕接受一片黃瓜。)
這是比較常規和比較容易辨識的虛擬語氣，希望同學們多加練習。希望廣大的考生在復習的過程中把遇到的所有長難句都匯總到一個筆記本上，積累自己的薄弱點，才能針對性提高。預祝考生們都能考試順利。
