l. 時間選擇型
當虛擬語氣用來表示壹種不能成為現實、不真實的情況時，須選擇與現在、將來和過去情況相反的三種不同時間，從而決定謂語動詞。這類虛擬語氣主要用於如下幾方面:
（1）虛擬條件句（含蓄虛擬條件句）
If l were you, l would study much harder．（與現在情況相反）
If he did（were to do，should do）it，he would do it well．（與將來情況相反）
I wouldn't have finished the work without your help． （與過去情況相反）
（2）由 even if (though) 引導的讓步狀語從句，其謂語動詞變化同於虛擬條件句，在口語中也可用直陳語氣。
Even if he were here, he wouldn't be able to help you． （與現在情況相反）
Nothing could have saved your father，even if the doctor had arrived on time．（與過去情況相反）
（3） wish後的賓語從句
I wish I were five years younger than you．（與現在情況相反）
He wishes you would go with him.（與將來情況相反）
I wished I had won the match．（與過去情況相反）
註意：wish的時態不影響從句三種時間的判定。
（4） as if (though)引導的表語從句和方式狀語從句
He looks as if he were very sleepy．（與現在情況相反）
They are talking about something, as if they were quarrelling with each other．
2．should+動詞原形型
這種類型的虛擬語氣壹般用於主語從句、賓語從句、表語和同位語從句。謂語由should+動詞原形構成，無論什麽人稱都不能用 would, should在口語中可以略去。
（1）在主語從句中用以表示驚奇、惋惜、理應如此，構成 It is natural （important, necessary, strange, a pity, a shame等）that… 句型。
It is natural that you should say apology to him．
It is a pity that you should be careless．
（2）用於表示命令、建議壹類動詞後的賓語從句，如：suggest，move，insist，demand，request，command等。
The young man insisted that he should be sent to do the work．
I suggested that we should set off earlier．
應註意：當insist表示對已發生過的事物的看法，或當suggest為“說明了”，“