1) 語氣：
陳述，疑問，祈使，虛擬
2) 虛擬語氣：表示一種純然假想的情況或主觀願望，動詞需要用一種特殊形式。
形式分為三類：現在虛擬語氣(動詞用原形)，過去虛擬語氣(動詞用過去式，但動詞be要用were)，過去完成形式(動詞用過去完成時)。有些情態動詞，如should, would, might常用在虛擬語氣中。
3) 虛擬語氣在條件句中的.用法：
虛擬條件句：就是包含條件從句的句子。
條件句分為：
真實條件句——有些條件句是可能實現的。
虛擬條件句——有些條件句是純假象的句子。
If we leave now, we can still catch the train.
If I had enough money, I would buy a computer.
If you are waiting for a bus you’d better join the queue.
4) 表示現在和將來情況的虛擬條件句：
⑴表示現在或將來的虛擬條件句——從句中的謂語(用過去式形式);主句謂語(would+動詞原形)
If I had a map I would lend it to you.
If you left your bike outside, someone would steal it.
If I were you I’d plant some trees round the house.
注：If I were you句中，were不能改成was,但在第三人稱單數後用was也可以。如：
If Helen were(was) here how nice it would be!
⑵主句謂語有時也可以由might, could構成。
If I could type, I might save a lot of time.
If he had a permit he could get a job.
⑶在條件句從句中有時還可以用were to +動詞，或should+動詞。
If he were (was) to resign, who would take his place?
If I should be free tomorrow, I will come to see you.
⑷在某些條件從句中(如were to +動詞，should+動詞)，if有時可以省略，這時語序要改為倒裝。
Were I Tom, I would refuse.
Were I in your shoes I would let him know what I thought of him.
Should she be interested, I’ll phone her.
⑸從句有時可以用If it were not for這種句型，表示“若不是”。
If it were not for your help, we would be in serious trouble.
