7.某些主语从句中的谓语动词要用虚拟语气

某些表示命令，要求，决定，建议，主张以及“重要性”和“紧迫性”等概念的主语从句中谓语动词要用虚拟语气。这时，谓语动词用动词原形或should+动词原形。这类主语从句一般由“It is(was)+形容词/过去分词+that引导的从句”构成。该结构中，常用的形容词主要有：advisable,appropriate,desirable,essential,imperative,important,necessary,obligatory,proper,urgent,vital等;常用的过去分词主要有：arranged,decided,demanded,desired,ordered,proposed,recommended,requested,suggested等。

It is necessary that you(should)stop smoking so heavily.

It is arranged that you(should)enter for this competition.

8.as if/though引起的从句

当as if/though引起的从句所表达的内容完全与实际情况相反或者纯粹是一种假设时，通常要用虚拟语气。如果从句表示与现在的事实相反，谓语动词用一般过去时;如果从句表示与过去的事实相反，谓语动词用过去完成时;如果从句表示与将来的事实相反，谓语动词用would(might,could)+动词原形;

They talked as if they had been friends for years.

I remember the whole thing as if it happened yesterday.

It looks as if it might rain.

Note:如果as if/though引起的从句所表达的内容被看作是事实或者有可能是真实的，则要用陈述语气。

It seems as if it is going to rain.

The meat tastes as if it has already gone bad.

9.lest,for fear that和in case引起的从句

当lest,for fear that和in case表示“以免，以防，生怕，惟恐”等时，在它们引起的从句中，谓语动词常用should+动词原形。

He ran away lest he should be seen.

He’s working hard for fear that he should fall behind.

He left early in case he should miss the last train.

10.If only引出的从句

If only引出的从句用以表达感叹性的愿望，常译为“要是…就好了”。If only从句经常省略结果主句，且主要用在虚拟语气中，即从句的谓语动词用一般过去时表示现在或将来不可能实现的愿望，或用过去完成时表示过去没有实现的愿望。

If only the rain would stop.

If only I’d listened to my parents.

Note:if only引出的句子偶尔也可使用陈述语气，但考生须注意的是，在各类测试中一般都以用虚拟语气为正确答案。

11.would rather(that)引出的从句

would rather意为“宁愿”，接从句时常省略关系代词that。would rather后的从句要用虚拟语气，即从句谓语动词用一般过去时表示现在或将来的愿望，用过去完成时表示与过去事实相反的愿望。

I’d rather you told me the truth.

I would rather you came tomorrow than today.

I’d rather you hadn’t told me about it.

12.It is(about/high)time(that)句型

该句型表示“(早)该做…”，其后的定语从句中的谓语动词要用虚拟语气表示。在测试中从句的谓语动词用一般过去时。

It is time that we went to bed.

It is high time that somebody taught you to behave yourself.

13.表示猜测的几种不同的方法及意义

一些情态动词与动词的完成式连用能够表示对过去情况的猜测或者未实现的可能性。

1)could have+过去分词

A.表示对过去可能发生的事情的推测，意为，“可能做了某事”。

He couldn’t have seen her yesterday.

They could have lost their way.

B.表示某事在过去本有可能发生，但事实上并未发生，意为，“本可以做某事”。

We could have started a little earlier.

I could have killed her.It was a narrow escape.