as if和as though引導的從句所敘述的情況與事實相反時，從句的謂語動詞多用虛擬語氣。在虛擬語氣結構中，用________時態來表示現在的情形；用________時來表示過去的動作或狀態；用________時表示正在進行的動作；用________時表示將要發生的動作。

當as if和as though從句的主語和主句主語一致，且從句中含有系動詞be的某種形式時該主語和系動詞可以省略。

例如：John paused as if(he was)expecting Mary to speak．

理解下在句子，翻譯句中劃線部分，并歸納其用法：

His exam results were not very good, but we must take into account his long illness．________

理解下在句子，翻譯句中劃線部分，并歸納其用法：

On no account you should attempt this exercise if you're seriously ill．(判斷正誤)________

理解下在句子，翻譯句中劃線部分，并歸納其用法：

Can political friction between the two nations account for the outbreak of the war?________

理解下在句子，翻譯句中劃線部分，并歸納其用法：

Imports from Japan account for 40％ of the total．________