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.
(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，但是真實情況並不是這樣，所以用虛擬語氣。
一般有2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 動詞的過去式爲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 )(查看爲世界盃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