概念
    語氣有三種:陳述語氣,祈使語氣和虛擬語氣.語氣表示說話人對勸詞所示示的動作或所處的狀態(tài)持有的態(tài)度或看法.
    二.相關(guān)知識點精講
    1.辨別if 引導(dǎo)真實條件句和if引導(dǎo)的虛擬條件句的區(qū)別
    If he has time, he will go with us.= Probably he has time and will go with us.
    If he had time, he would go with us.=But in fact he has no time.
    2.虛擬條件句中主句和從句的謂與動詞構(gòu)成形式如下表

	if條件句中的謂與動詞	主句的謂與動詞
與現(xiàn)在的事實相反	1. 行為動詞用did 形式     2. be動詞用were	should     would     could   + 動詞原形     might
與過去的事實相反	had + done	should     would     could   + have + done     might
與將來的事實相反	1. 行為動詞用did     2. should + 動詞原形     3. were to + 動詞原形	should     would     could   + 動詞原形     might

3.混合時間的虛擬語氣
    如果條件句中的動作和主句的動作不是同時發(fā)生，主句和從句的謂語動詞的形式應(yīng)分別根據(jù)各自所表示的時間加以調(diào)整。
    1)0If I had received the passport yesterday, I would start today.
    2)If he had telephoned me last night, I would see him now.
    3)If he had followed the doctor’s advice, he would be all right now.
    4)If China had not been liberated, the working people would still be leading a miserable life.
    4.should/ could / might/ ought to + have done 表示“過去本應(yīng)該/可以做而實際上卻沒做”
    needn’t have done 表示“過去沒必要作而實際上做了”