1.陳述語氣：陳述一個事實或提出一種想法的語氣。I don’t know anything about the accident.
2.疑問語氣：用來提出問題或詢問情況的語氣。Do you have a camera?
3.祈使語氣：用來向對方提出請求，邀請，給予忠告，指示，警告，命令等的語氣。在祈使語氣中，直接用動詞原形，主語you一般省略。Come in and take a seat.
4.虛擬語氣：表示一種假想或主觀愿望。If I were you , I would forgive Mary.
這種條件句實現的可能性非常大，常用 If, unless, once, as/so long as, on condition that來引導。
If I have time, I will go to America.
Unless they invite me, I will not go to the party.
If I were you, I would choose to work in a small town.
If the weather were fine now, we would have a picnic.
If I had a map now, we could easily get out of the forest.
If you had come yesterday, you would have met Tony.
If it had not been for your help yesterday, I could not have caught the bus.
對將來情況的虛擬，條件句中謂語動詞可用should do, were to do或動詞的過去式三種形式，主句謂語用“would/should/might/could+動詞原形”。
If I were to do the job, I would do it in a different way.
If you should have any difficulty in getting the information, you could come directly to my office. （事實是：你可能沒有太大困難）
If it snowed tomorrow, our plan would be postponed.
If you had taken my advice then, you would not be in trouble now.
?。╥f條件句是對過去的虛擬，故用過去完成時，而主句是對現在的虛擬，故用would do 形式）
If she were to leave , I would have heard about it.
?。╥f條件句是對將來的虛擬，用were to do形式，而主句是對過去的虛擬，故用would have done形式）
有時虛擬的條件并不是直接通過條件句來表示，而是用一些詞或短語如without, but for, otherwise等或上下文暗示來說明，這種句子叫含蓄虛擬語氣。
Without electricity (=if there were no electricity), the world would be dark.
But for your advice(=if it had not been for your advice), I would not have made such great progress.
虛擬條件句中有had, should, were時，可以把if去掉，然后把had, should, were提到主語前面，形成部分倒裝。
If time were to go back/ Were time to go back, I would work harder.
If I had worked hard/ Had I worked hard, I would have surely succeeded.
If it should be fine tomorrow/Should it be fine tomorrow, we would go for an outing.
I wish I lived on the seashore.
I wish you had come to the wedding yesterday.
I wish I could know the answer to the question tomorrow.
1.2 would rather后的賓語從句
I would rather you did not smoke.
I would rather you had not let out the secret.
表示“建議，命令，要求”等動詞后的賓語從句謂語常用“should+動詞原形”，有時候should可以省略。常這樣的動詞有insist, demand, desire, require, request, propose, suggest, advise, command, order, recommend.
I suggest you (should) have enough sleep.
My parents advise I (should) not watch too much TV.
We suggest that Tony (should) sing another song.
It is/was important(necessary, appropriate, proper, vital) that...
It is/was unnatural(unthinkable, strange, surprising) that...
It is/was suggested(requested, required, ordered, proposed,decided) that..
3.1 as if, as though引導的表語從句中的虛擬語氣
as if, as though引導的表語從句中，用一般過去時表示與現在事實相反，用過去完成時表示與過去事實相反。
It looks as if he were drunk.
He seemed as if he had suffered a lot.
當句子的主語是表示“建議，要求，命令，主張”的名詞，如suggestion, proposal, advice,demand,order,requirement等，其后的表語從句謂語常用“should+動詞原形”，should可以省略。
Our decision is that the meeting (should) be put off.
My suggestion is that we (should) walk home instead of taking a taxi.
表示“主張，要求，建議，命令”名詞如suggestion, proposal, advice, demand, order, requirement等后的同位語從句的謂語常用“(should+)動詞原形”結構。
He made a suggestion that they (should) hold an English speech contest.
The requirement that students (should) learn to protect themselves is successfully carried out in most schools.
as if, as though引導狀語從句有時也可表示與事實相反的情況。用一般過去時表示與現在事實相反，用過去完成時表示與過去事實相反。
He speaks English as if he had learned it in America.（事實是：他的英語不是在美國學的）
She closes her eyes as if she was tired.（事實是：她并不累）
引導目的狀語從句的短語為in order that, so that, for fear that等，從句中謂語常用“should（may, might, can, could, would等）+動詞原形”。
He raised his hand up so that he could be seen.
He left early in order that the children would not be alone in the house.
1.It is (high) time that...句型
It’s time that we started off. 我們該出發了。
It’s high time that we should have a rest. 我們該休息了。
2.If only 句型
If only I were younger. 要是我年輕點就好了。
If only I had written his phone number. 要是我記下他的電話號碼就好了。