虚拟语气主要由 if 引导的状语从句和主句构成的

第一节 纯条件的虚拟语气

• 结构：If + 主语 + 一般现在时动词, 主语 + will/may/can/should/shall/must/ought to + 原形动词

• 表示从句条件成立，主句的动作就会发生；但是从句条件是否成立及什么时候可以确定是否成立都未知

  ​ 比如：当他做完手头的事，他就回来了。他现在有没有做完事，我们不知道；他什么时候会做完，我们也不知道

• 除 if 外，once、when、before、as soon as、unless 也可在纯条件的虚拟语气中使用

• 通常助动词放在主句中，不放在 if 从句中。但是如果 if 从句主语为人时，可用 can、will、must 等助动词

• 纯条件虚拟语气主句中的主动词均为一般现在时，但 should 是例外，此时它等价于 ought to，译为“应当”

第二节 与现在事实相反的虚拟语气

• 结构：If + 主语 + 一般过去时动词, 主语 + would/could/might/should/ought to + 原形动词

• 表示明确知道与现在的情况相反

• 无论主语是第几人称，if从句 中的 be动词，始终为 were

  ​ If I were you, I wouldn't do it.

第三节 与过去事实相反的虚拟语气

• 结构：If + 主语 + had + 过去分词, 主语 + would/could/might/should/ought to + have + 过去分词

• 表示明确知道与过去的情况相反

  ​ If he had had money, he would have bought a car.

第四节 与将来状况相反的虚拟语气

• 因为将来还没发生，其实我们没办法确定是否会与将来的状况一致，因此这种虚拟语气分低可能性和高可能性

• 低可能性结构：If + 主语 + should + 原形动词, 主语 + would/could/might/should/ought to + 原形动词

  If you should fall ill, the meeting would be put off.

• 高可能性结构：If + 主语 + should + 原形动词, 主语 + will/can/may/should/ought to + 原形动词

  ​ If you should fall ill, the meeting will be put off.

• 注意：if从句 中必须有助动词 should

• 表与将来状况相反的虚拟语气，可与祈使句主句连用

  ​ If I should be late, be sure to wait for me.

第五节 使用虚拟语气应注意的事项

表与真理相反的虚拟语气

• 结构：If + 主语 + were to + 原形动词, 主语 + would/could/might/should/ought to + 原形动词

  ​ If the sun were to rise in the west, how surprised these sunflowers would be.

if 的省略

在含有 had、should、were 的虚拟语气中，可将这些词提到主语前面，然后省略 if

• 与过去事实相反，含有助动词 had

  ​ If he had done it, he would have felt sorry. -> Had he done it, he would have felt sorry.

• 与将来事实相反，含有助动词 should

  ​ If he should tell lies, I would punish him. -> Should he tell lies, I would punish him.

• 与现在事实相反，含有助动词 were

  ​ If he were lazy, he might fail. -> Were he lazy, he might fail.

时态不一致的假设

if从句与过去事实相反，用过去完成时；主语与现在事实相反，用一般过去时

• 结构：If + 主语 + had + 过去分词, 主语 + would/could/might/should/ought to + 原形动词 + now/today

  ​ If I had started saving then, I would be able to buy a car now.

可取代 if 的其他连词

除 if 外，如：in case、on condition that、provided/providing (that)、so long as、as long as 等也有与 if 等价的效果，但是它们只能用于表纯条件的虚拟语气，也就是说，其引导的从句时态只能为一般现在时。在纯条件的虚拟语气一节中还提到了其他可以其他可以替代 if 的词。

当这些词替代 if 时，整句将不必遵循纯条件的虚拟语气的结构

​ I will take the trip with you on condition that you bear the express.

but for

译为“若非、要不是...”

• 表与现在事实相反，译为“若非...就...”

  • 结构：But for + 名词, 主语 + could/would/might/should/ought to + 原形动词

    ​ But for his hard work, I wouldn't like him.

• 表与过去事实相反，译为“若非当时...就...”

  • 结构：But for + 名词, 主语 + could/would/might/should/ought to + have + 过去分词

    ​ But for the money (which) he lent me, I could not have bought that book.

• but for 在还可以用 without 替代，两者用法是完全一致的

• 可以看出，表示肯定的虚拟语气，如：如果...就能...，用一般的表示就好；但如果表示否定的虚拟语气，如：如果不...就...，用 but for 比较方便。

lest ... (should) ...

lest 译为“以免 ...”，当 lest 引导状语从句时，助动词必须用 should，但 should 通常可以省略。

​ You must study hard lest you (should) fail the exam.

该句型等价于:

• for fear + 主语 + may + 原形动词(表现在或将来的状况)
• for fear + 主语 + might + 原形动词(表过去的状况)
• for fear of + 动名词

as if、as though

两者均可引导状语从句，译为“仿佛...”，且遵循：

• 表极大可能，用一般现在时

  ​ It looks as if/though it is going to rain.

• 表与现在事实相反，用一般过去时

  ​ Mr. Wang loves me as if/though I were his own child.

• 表与过去事实相反，用过去完成时

  ​ He looked as if/though nothing had happened.

• 接不定式表即将发生的状况

  ​ She opened her lips as if/though (she was going) to speak.

What if ... should ... ?

表对将来状况存疑的虚拟语气，译为“要是...的话会怎样/要怎么样?”，if从句中一般使用 should。

​ What if he should come? -> What might happen if he should come?

What if 后面也可以使用一般现在时

​ What if he comes? -> What shall we do if he comes?

It is time + that从句

It is time/about time/high time + that从句(用一般过去时)，译为“该是...的时候了”

​ It is time (that) he went to bed. 等价句型 It is time for him to go to bed.

事实上他现在还没去睡觉，所以该句是与现在事实相反的虚拟语气，因此用一般过去时。

if only ...

该句型只用于与现在事实和过去事实相反的虚拟语气中，因此时态只能用一般过去时或过去完成时。译为“要是...就好了”

​ If only he were here.

wish 和 hope

• wish 和 hope 均可用不定式作宾语

  ​ I wish to travel around.

• 也都可以做不及物动词，用 for 作介词，译为“期望获得”，以名词作宾语

  ​ I wish for a chance to try it.

• 当表“希望”时，wish 和 hope 都可以使用，但表“祝福”时，只能用 wish

  ​ I wish you a good time.

• hope 不能直接表“祝福”，要与含 助动词may 的 that从句 形成祈使句，且 I hope that 通常省略，然后将 may 倒装

  ​ I hope that he may live long. -> May he live long.

• 当以 God 为主语时，may 也可以省略

  ​ I hope that God may bless you! -> May God bless you! -> God bless you!

• 表“国王、国家万岁”时有另一种倒装方法

  ​ I hope that the king may live long! -> May the king live long! -> Long live the king!

• wish 和 hope 都可以接 that 从句，但 wish 接 that从句 表虚拟语气，hope 接 that从句 不表虚拟语气

• wish 后接 that从句时，与现在事实相反用一般过去时，与过去事实相反用现在完成时，不能表与将来状况相反，因此不会出现一般现在时

• hope 后接 that从句时，根据要表达的意思选择时态

表对过去事实的猜测

• must have + 过去分词，译为“一定曾经 ...”

• May have + 过去分词，译为“可能曾经 ...”

• cannot have + 过去分词，译为“不可能曾经 ...”

• 如：He looks tired; he may have stayed up late last night.

• 用疑问句表猜测时，要用 can ... have ... ? 句型

  ​ Can he have done it?

意志动词

• 表“建议”：suggest、recommend、advise、urge、propose、move

• 表“要求”：ask、desire、demand、require、request、insist

• 表“命令”：order、commend

• 表“规定”：rule、regulate、stipulate

• 表“主张”：advocate、maintain

• 使用上述动词时，需要用 that从句 作宾语，并其中用 should 作助动词，且should 通常省略

  ​ They suggested that he (should) come home on time.

• 当 suggest 表“暗示”；insist 表“坚信”；maintain 表“坚决认为” 时，that从句 用一般时

  ​ His words suggested that he wasn't telling a lie.

• insist 作不及物动词时，与介词 on 搭配，译为“坚决要”，表“固执”的意思，而 persist in，也译为“坚持要”，但是表“不轻易放弃”的意思

• that从句 若是置于上述意志动词所变的名词后面，且与该名词成同位语时，that从句 要用助动词should，但should常常省略

  ​ It is necessary that he (should) study abroad.

表“有必要的”形容词

• 表有必要的形容词通常有：necessary、important、essential、imperstive、urgent、desirable、recommendable、advisable

• 这些形容词修饰 that从句 时，通常使用助动词should，但should往往省略

  ​ It is necessary that he (should) work hard.