1.虛擬語氣用在 suggest, demand, insist, ask, move, advise. desire, order, propose, require, recommend, request, maintain, vote 等表示建議，命令，請求的動詞後的賓語從句中，從句的謂語用 should + 原形動詞，但美國英語中常要省去 should 。例如：
I suggest we start at once.
They propose that you (should) read the rules carefully.
The commander ordered that the soldier fire at the enemy.
He insisted that he go there on foot.
2.虛擬語氣用在 It is + 形容詞（或過去分詞）+ that... .的主語從句中，表示要求，建議，提議，命令，請求等，英國英語中從句的謂語用“should +原形動詞（或完成式）構成”，而美國英語中則要求用原形動詞。該句型中常用的形容詞或過去分詞有：advisable, desirable, essential, important, better, keen, natural, necessary, probable, possible, strange, vital, desired, demanded, requested, suggested, recommended, required 等。例如：
It is necessary that the machine (should) be oiled once a week.
It is important that everyone (should) obey the law.
3.虛擬語氣用在與表示建議，要求，提議，命令，請求等意思有關的同位語從句和表語從句中，英國英語中要求從句謂語用“ should + 原形動詞”，而美國英語中則要求用原形動詞， 但在 as if, as though引導的表語從句中，從句中謂語用動詞過去式或過去完成式，表示對現在或過去的虛擬。例如：
He complied with the requirement that the experiment (should) be done tomorrow.
他同意明天再做實驗的要求。
He looks as if he were tired.他面帶倦容。
We agree to the suggestion that they (should) work on night shifts in turn.
我們同意輪流值夜班的建議。
4.虛擬語氣用在 It is (high, about) time (that) 句型中，定語從句中謂語用動詞過去式，意指將來。例如：
It is time (that) we went home.
It is high time we had the meeting.
It is time you left.
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