主语从句中的虚拟语气问题
■ it is important that…类句型中的主语从句用虚拟语气
在It is important that...句式中，其中由that引导的主语从句通常用“should+动词原形”这样的虚拟语气，其中的should在美国英语中通常省略。如：
It is important that he work hard. 他用功是很重要的。
It is important that he should know about this. 他必须知道此事。
It’s important that we (should) reply to her letter. 回复她的信是重要的。
It is important that we should speah politely. 我们说话要有礼貌，这是很重要的。
注：与本句型相似的还有以下句型：
1. it is necessary that…句型：
It is necessary that I should return it this morning. 今天早上我就有必要还回去。
It is not necessary that everyone be a scientist. 没有必要使每一个人都成为科学家。
2. it is impossible that…句型：
It is impossible that he should go home. 他不可能会回家去。
3. it is essential that…句型：
It is essential that the meeting start at eight. 会议务必在8点钟开始。
It is essential that we should tell her the news. 我们有必要告诉她这个消息。
4. it is desirable that…句型：
It is desirable that everything should be ready by five o’clock. 最好一切在5点前准备就绪。
It is desirable that there should be a cold season once a year. 每年有一个寒冷的季节，是很令人盼望的。
5. it is advisable that…句型：
It is advisable that everyone should have a map. 最好每一个人都有一张地图。
It is advisable that he should see a dentist regularly. 他定期去看牙科医生是明智的。
■ it is strange that…类句型中的主语从句用虚拟语气
在It is strange that...句式中，其中由that引导的主语从句可以陈述语气，也可以用虚拟语气，其区别是：用虚拟语气（should+动词原形）主要强调惊讶之意，其感情色彩比较浓；若不特别强调感情色彩，则用陈述语气。比较：
It’s strange that there are no lights on. 真奇怪，没有一盏灯是开着的。
It’s strange that there should be no lights on. 真奇怪，竟然没有一盏灯是开着的。
It’s strange that he has left without telling us. 奇怪，他没有和我们说一声就走了。
It’s strange that he should have left without telling us. 奇怪，他竟然没有和我们说一声就走了。
注：与本句型相似的还有以下句型：
1. it is natural that…句型：
It’s natural that you should forget it. 你忘了这事不足为奇。
It is quite natural that he should think so. 他这样想是很自然的事。
2. it is amazing that…句型：
It is amazing that they’re still together. 他们还在一起，真是让人感到惊讶。
It is amazing that they should still be together. 他们竟然还在一起，真是让人感到惊讶。
3. it is astonishing that…句型：
It’s astonishing that she says that sort of thing to you. 她对你说出那种话，真叫人吃惊。
It’s astonishing that she should say that sort of thing to you. 她居然对你说出那种话，真叫人吃惊。
4. it is surprising that…句型：
It is surprising that she has married a farmer. 她和农民结婚实在令人意想不到。
It is surprising that she should have married a farmer. 她居然和农民结婚实在令人意想不到。