If + did / were + ..., ... would/ should/ could/ might + do （動詞原形）
If I were you, I would go abroad at once. (I am not you.)
If he knew it now, he could help me. (He doesn't know it now.)
If + had done + ..., ... would (might) have done ...
If I had known your telephone number yesterday, I would have phoned you. (I didn't know your telephone number.)
If you had come here a little earlier just now, you might have met her. (You didn't come here earlier.)
(1) If + should + v., ... would + v. （可能性很小）（譯作「萬一」）
If it should rain tomorrow, you could stay at home.
If I should fail, what should Ido?
(2) If + did / were to + v ..., would + v. （完全不可能）
If the sun were to rise in the west, I would lend you the money.
If you finished it in 3 minutes, I would give you my car.
● 下列動詞后的「賓語從句」中需要用虛擬形式，即should + 動詞原形，shoud在美國英語中要省略（TOEFL語法考點）。此類常見的動詞有：order, ask, decide, demand, require, recommend, suggest（建議）, insist（堅決要求）, advise, etc.
He suggested that we (should) help them with English.
The teacher ordered that the homework (should) be finished within half an hour.
● 下列名詞后的同位語從句中要用「should + 動詞原形」（should可省去）的虛擬。此類常見的名詞有：suggestion, order, request, demand, importance, proposal.
He made a suggestion that we (should) have a fancy dress party.
I think it is a thing of importance that it (should) be done soon.
● It is / was important / necessary / natural / essential / advisable / strange / surprising, ect. + that +主語+ should + v.
It is strange that you should say such a thing.
It was important that you should tell me all the information.
● wish后的賓語從句中，as if 后的狀語從句中，須用下列的虛擬形式：
主 + wish+ (that) + 主 + did / were（指現在）
I wish that I met my uncle now.
I wish I had met my uncle yesterday.
I wish I could meet my uncle tomorrow.
● It is (high) time that ... + did / were ...
It is time that you went to bed.
● would rather that ... + did / were ...
I would rather that you were not here now.
● would sooner that ... + did / were ...
I would sooner that you got up earlier.
I would sooner that you were not my brother.