一、虛擬語氣是什么？

虛擬語氣用來表示假想，而不表示客觀存在的事實，所說的是一個條件，不一定是事實，或與事實相反。虛擬語氣通過謂語動詞的特殊形式來表示。英語中的語氣分為陳述語氣、祈使語氣、疑問語氣、虛擬語氣四類。虛擬語氣表示說話者所說的話并非事實，或難以實現的，甚至表達相反的概念。另外表示主觀愿望或表達某種強烈的感情時，也用虛擬語氣。

二、用法及動詞形式

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

例句：1. If I were you, I would take an umbrella.
如果我是你，我會帶把傘。（事實：我不可能是你）
2. If I knew his telephone number, I would tell you.
如果我知道他的電話號碼，我就會告訴你。（事實：不知道）
3. If there were no air or water, there would be no living things on the earth.
如果沒有水和空氣，地球上就不會有生物。（事實：地球上既有空氣也有水）
4. If I had any money with me,I could lend you some.
如果我帶錢了，我就會借給你些。（事實：沒有帶錢）
5. If he studied harder,he might pass the exam.
如果他再努力些，就能通過考試了。（事實：沒有努力）
6. She looked at me as if I had been a stranger.
她看我的樣子好像我是一個陌生人。（事實：我并非陌生人）

與過去事實相反：從句：If 主語+had+done，主句：主語+should/would/could/might+have done

例句：1. If I had got there earlier, I should/could have met her.
如果我早到那兒，我就會見到她。（事實：去晚了）
2. If he had taken my advice, he would not have made such a mistake.
如果他聽我的勸告的話，就不會犯這樣的錯誤了。（事實：沒有聽我的話）

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

例句：1. If he should come here tomorrow, I would talk to him.
如果他明天來這兒的話，我就跟他談談。
（事實：來的可能性很小）
2. If there were a heavy snow next Sunday, we would not go skating.
如果下周日下大雪，我們就不能去滑冰了。（事實：下雪可能性很小）
3. If she were to be here next Monday, I would tell her about the matter.
如果她下周一來這兒的話，我就會告訴她這件事的始末。（事實：來的可能性很小）