1、If條件句中絕對不可出現(xiàn)“would”。
2、根據(jù)句中的時(shí)間狀語，有時(shí)可能出現(xiàn)“混合虛擬”的情況，即主句可能是現(xiàn)在的情況，條件句也許是發(fā)生在過去的情況，但都是遵守上述句型。
3、在條件句中如果出現(xiàn)were， had， should可省去if，將主語與這些詞倒裝，例如：
Had the doctor come last night， the boy would have saved.
Were I to go to the moon one day， I would see it with my own eyes.
Should it rain tomorrow， we would stay at home.
二、名詞性虛擬語氣：
在表示命令、建議要求、驚嘆時(shí)的名詞性從句中需用虛擬語氣，基本句型：主語＋（should）＋動(dòng)詞原形，如：Mother insisted that John go to bed at 9 o’clock.（賓語從句）
We suggested that the meeting should not be held.
It was required that the crops should be harvested at once.（主語從句）
The suggestion that he be invited was rejected.（同位語從句）
That is their demand that their wages be increased.（表語從句）
注意：在這種句子中絕不出現(xiàn)“would”“must”“could”等。