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完成时态：只在必要的时候使用
只要一般时态可以说清楚就不用完成时态。GMAT偏爱简单。
Joe learned about an epoch in which dinosaurs had walked the earth. 这里使用过去完成时是没有必要的，因为the earlier action does not have a direct bearing on the context of the later action. Thesequence of time does not need to be clarified or emphasized.Thus, 过去完成时在这句话中是错误的。
使用完成时的原则：you should use the perfect tenses only when you can justify them with the rules:
当一个动作发生在过去，且一直持续到现在，或其影响一直持续到现在，使用现在完成时，
当一个动作发生在另外一个已经发生的动作之前，且需要强调这两个动作的先后顺序时使用过去完成时。其他情况下只用简单的时态。
e.g. The scientists announced thatthe supercollider was ready, that it had not cost too much, and that it would provide new insights into the workings of the universe.
小结： conditional tense: would + verb 过去时+conditional tense
The scientists believedthat the machine would be wonderful. 现在时+will
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The scientist believes that the machines will be wonderful. 注意不要mix them
虚拟语气
Indicative mood：陈述语气 Imperative mood:祈使句、命令句
虚拟语气（subjunctive mood）的两种使用：
(1) unlikely or unreal condition(if, as if , or as though或类似的句子
之后)
(2) proposals, desires, and requests formed with certain verbs and the
word that.
The Hypothetical subjunctive假设虚拟
表达unlikely or unreal condition(if, as if , or as though或类似的句子之后)
To overcome my fear of germs, I will think about diseaseas thoughit were harmless.
说话人并不相信疾病实际上是无害的，他只是虚拟假设认为疾病无害。 Were的使用： If I were a rich man, …… If…then…结构：记住如下的结构: (1) general rule with uncertainty
If Sophie eats pizza, then she becomes ill. if现在时，then现在时
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Whenever S eats pizza, she becomes ill.
(2) general rule with some uncertainty If S eats pizza, then she may become ill. If现在时，then can or may Uncertain outcome
(3) particular case with no uncertainty
If S eats pizza tomorrow, then she will become ill. If 现在时，then 将来时
另外一种可能性：If S has eaten pizza, then she will become ill.
(4) Unlikely case(in the future)
If S ate pizza tomorrow, then she would become ill.
含义是：作者认为S不太可能明天吃披萨，the conditional tense(would)shows the hypothetical result of an unlikely or untrue condition. Would可以用could 代替。
(5) Case that never happened(in the past) if past perfect, then conditional perfect.
上述5中if…then…表达方式可以涵盖gmat中出现的结构。
命令式虚拟语气
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Bossy verbs+that+subject+command subjective We propose that the school board disband.
注意不是每个bossy verb都用虚拟语气，一些BOSSY VERB不用虚拟语气，比如want等，具体哪些是习惯。
Command verbs that takedesire: Demand, dictate, insist, mandate, propose, recommend, request, stipulate, suggest
e.g. we demand that he be here.
Propose可以使用不定式结构，在没有第二个主语的情况下。 The attorneys proposed to meet the following day.
Verbs that take only the infitives: Advise, allow, forbid, persuade, want e.g. we allow him to be here.
Verbs that take either the command subjective or the infinitive. Ask, beg, intend, order, prefer, urge, require.
Prohibit from doing sth.
Bossy verbs 的变形也用虚拟语气，比如demand, request.
only the command subjunctive when indicating 35
His demand that he be paid full severance was not met.
一些形容词也用于虚拟语气：advisable, crucial, desirable, fitting, imperative, important, mandatory, necessary, preferable, urgent, vital同样，这些词后也可以跟不定式。
It is essential for Gary to be ready before noon. Whether之后不用虚拟语气
I like ice cream, whether it is chocolate, vanilla, or any other flavor. 一些BOSSY VERBS可以用于NON-BOSSY WAYS:
比如suggest，并不是一看到suggest就一定要用虚拟语气，要看具体含义： Her presence suggests that she is happy.这里suggests是means的含义。
主动语态与被动语态
Whoever actually performed the action in the sentence may follow the verb in a phrase headed by the prepositionby (by the hungry students). Use by only for the actual doers of the action. Use through or because of when you want to describe any instrument or means, which might be an awkward or nonsensical subject in active voice.
e.g. The pizza was accidentally eaten by a quirk of fate.(wrong) A quirk of fate accidentally ate the pizza.(wrong) Through a quirk of fate, the pizza was accidentally eaten.
如果BY这个词在文中非划线部分出现，则可以考虑使用被动语态。