1)虛擬語氣可以表示過去，現在和將來的情況，時態的基本特點是時態往后推移。

a. 與現在事實相反的假設

條件從句主句一般過去時(be用were)should(would)等 +動詞原形

例如：If they were here， they would help you. 如果他們在這兒，會幫助你的。

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

b. 與過去事實相反的假設

條件從句主句過去完成時should(would)等+ have+ 過去分詞

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

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

c. 表示對將來不大可能發生的事情的假想

條件從句主句一般過去時should/would等 + 動詞原形were+ 不定式should+ 動詞原形

例如：If you succeeded， everything would be all right. 如果你將來成功了，一切都會好的。

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.