1)虛擬語(yǔ)氣可以表示過(guò)去，現(xiàn)在和將來(lái)的情況，時(shí)態(tài)的基本特點(diǎn)是時(shí)態(tài)往后推移。

a. 與現(xiàn)在事實(shí)相反的假設(shè)

條件從句主句一般過(guò)去時(shí)(be用were)should(would)等 +動(dòng)詞原形

例如：If they were here， they would help you. 如果他們?cè)谶@兒，會(huì)幫助你的。

含義：They are not here， they can‘t help you.

b. 與過(guò)去事實(shí)相反的假設(shè)

條件從句主句過(guò)去完成時(shí)should(would)等+ have+ 過(guò)去分詞

例如：If he had come yesterday， I should / would have told him about it.如果他昨天來(lái)的話，我會(huì)把這件事告訴他的。

含義：He did not come yesterday， so I did not tell him about it.

c. 表示對(duì)將來(lái)不大可能發(fā)生的事情的假想

條件從句主句一般過(guò)去時(shí)should/would等 + 動(dòng)詞原形were+ 不定式should+ 動(dòng)詞原形

例如：If you succeeded， everything would be all right. 如果你將來(lái)成功了，一切都會(huì)好的。

If you should succeed， everything would be all right.

If you were to succeed， everything would be all right.

含義：You are not likely to succeed， everything will be what it is now.