虛擬語氣（Subjunctive Mood）用來表示說話人的愿望、請求、意圖、建議、懷疑、設(shè)想等未能或不可能成為事實(shí)的情況，以及在說話人看來實(shí)現(xiàn)可能性很小的情況。由于虛擬語氣是動詞的一種特殊表現(xiàn)形式，因而動詞的變化形式體現(xiàn)出虛擬語氣所包含的不同含義。
虛擬語氣用于非真實(shí)條件句時的構(gòu)成如下表所述：
If從句
主句
與現(xiàn)在事實(shí)相反
動詞的過去式（be為were）
would（should）＋動詞原形
與過去事實(shí)相反
動詞的過去完成式
would （should, could, might）+ have +動詞的過去分詞
與將來時間相反
1）動詞的過去式（be為were）
2）were +不定式
3）should＋動詞原形
would (should, could, night)+動詞原形
例句：
If I had time, I would go to the movies with you.
If we hadn’t made adequate preparations, the conference wouldn’t have been so successful.
If it were to rain tomorrow, the game would be put off.
虛擬語氣用于非真實(shí)條件句時的構(gòu)成如下表所述：
If從句
主句
與現(xiàn)在事實(shí)相反
動詞的過去式（be為were）
would（should）＋動詞原形
與過去事實(shí)相反
動詞的過去完成式
would （should, could, might）+ have +動詞的過去分詞
與將來時間相反
1）動詞的過去式（be為were）
2）were +不定式
3）should＋動詞原形
would (should, could, night)+動詞原形
例句：
If I had time, I would go to the movies with you.
If we hadn’t made adequate preparations, the conference wouldn’t have been so successful.
If it were to rain tomorrow, the game would be put off.