1) 语气：
陈述，疑问，祈使，虚拟
2) 虚拟语气：表示一种纯然假想的情况或主观愿望，动词需要用一种特殊形式。
形式分为三类：现在虚拟语气(动词用原形)，过去虚拟语气(动词用过去式，但动词be要用were)，过去完成形式(动词用过去完成时)。有些情态动词，如should, would, might常用在虚拟语气中。
3) 虚拟语气在条件句中的用法：
虚拟条件句：就是包含条件从句的句子。
条件句分为：
真实条件句——有些条件句是可能实现的。
虚拟条件句——有些条件句是纯假象的句子。
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) 表示现在和将来情况的虚拟条件句：
⑴表示现在或将来的虚拟条件句——从句中的谓语(用过去式形式);主句谓语(would+动词原形)
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,但在第三人称单数后用was也可以。如：
If Helen were(was) here how nice it would be!
⑵主句谓语有时也可以由might, could构成。
If I could type, I might save a lot of time.
If he had a permit he could get a job.
⑶在条件句从句中有时还可以用were to +动词，或should+动词。
If he were (was) to resign, who would take his place?
If I should be free tomorrow, I will come to see you.
⑷在某些条件从句中(如were to +动词，should+动词)，if有时可以省略，这时语序要改为倒装。
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从句有时可以用If it were not for这种句型，表示“若不是”。
If it were not for your help, we would be in serious trouble.
