It is a waste of time trying to explain．设法解释是浪费时间。

3)It is + useless (nice, good，interesting, expensive等形容词)+ doing sth． It is useless speaking.光说没用。

It is nice seeing you again．真高兴又遇到了你。 It is good Playing chess after supper．晚饭后弈棋挺好。 It is expensive running this car．开这种小车是浪费。

四． 名词性从句用法

名词性从句包括：主语从句、宾语从句、表语从句、同位语从句，多由连词that，wh- 疑问词或由what，whatever等关系代词引导，其中同位语从句在四级考试中最常见。 1. 主语从句

1) 主语从句在句子中充当主语，句子的谓语动词用第三人称单数。如： Why he refused to work with you is still mystery. Whoever comes is welcome.

2) 主语从句前的that不能省略，仅起连接作用，有时为了保持句子平衡，that从句后置，而由it作形式主语。如：

That he became a lawyer may have been due to his mother’s influence. It is not true that he has moved to New York. 2. 宾语从句在句子中充当宾语，如：

I don’t know where the sound came from.

Don’t be satisfied with what you have achieved.

3．当从句放在系动词 be, look, remain, seem等后即构成表语从句。 The trouble is that I have lost his address. It seemed that the night would never end. 4. 同位语从句

1) 同位语从句是对与之同位的名词中心词作进一步解释，能接名词性从句的常见名词有: idea, fact, news, belief, hope, evidence, opinion, problem, truth, answer, proposal, theory, decision, discovery, problem, thought, understanding 等。 The news that he has passed the examination is exciting.

2) 同位语从句与定语从句的区别在于：定语从句是对先行词加以修饰、限制，而同位语从句时说明名词中心词的具体内容；that在定语从句中充当成分，为关系代词，而在同位语从句中不做任何成分,仅起连词作用。如：

The fact that we talked about is very important. (定语从句)

The fact that he succeeded in the experiment pleased everybody.( 同位语从句) 5. whether与if 在名词性从句中的用法区别

if一般只用于引导宾语从句，而 whether可引导包括宾语从句在内的其他名词从句。 1) 连词whether引导的主语从句，表语从句不能用if来替换。 Whether he will attend the meeting is uncertain. The question is whether he will come.

2) 宾语从句中，whether常与or not搭配，可以说whether or not，而不说if or not。 I don’t care whether or not she will attend the meeting. 3) whether可用在介词后，或带to不定式前，if则不可。 She doesn’t know whether to get married now or wait. I haven’t settled the question of whether I’ll go back home. 4）某些动词后面(如discuss)只能用whether ，不用if。 We discussed whether we should go on climbing. 另外补充

名词性从句是由if, whether, that 和各种疑问词充当连接词所引导的从句，其功同名词一样。 一．主语从句

主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。

1. It 作形式主语和it引导强调句的比较

It 作形式主语代替主语从句，主要是为了平衡句子结构，主语从句的连接词没有变化。而it引导的强调句则是对句子某一部分进行强调，无论强调的是什么成分，都可用连词that。被强调部分指人是也可用who/whom。例如： It is a pity that you didn’t go to see the film. It doesn’t interest me whether you succeed or not.

It is in the morning that the murder took place. It is John that broke the window. 2. 用it 作形式主语的结构 (1) It is ＋名词＋从句 It is a fact that …事实是… It is an honor that…非常荣幸

It is common knowledge that…是常识 (2) it is ＋形容词＋从句 It is natural that…很自然… It is strange that…奇怪的是… (3) it is ＋不及物动词＋从句 It seems that…似乎… It happened that…碰巧… (4) it ＋过去分词＋从句 It is reported that…据报道… It has been proved that…已证实… 3. 主语从句不可位于句首的五种情况 (1) if 引导的主语从句不可居于复合句句首。

(2) It is said , (reported) …结构中的主语从句不可提前。例如：

It is said that President Jingo will visit our school next week. (right) That President Jiang will visit our school next week is said. (wrong) (3) It happens…, It occurs… 结构中的主语从句不可提前。例如： It occurred to him that he failed in the examination. (right) That he failed in the examination occurred to him. (wrong)

(4) It doesn’t matter how/whether …结构中的主语从句不可提前。例如： It doesn’t matter whether he is wrong or not. (right) Whether he is wrong or not doesn’t matter. (wrong) (5) 含主语从句的复合句是疑问句时，主语从句不可提前。例如： Is it likely that it will rain in the evening? (right) Is that will rain in the evening likely? (wrong) 4. What 与that 在引导主语从句时的区别

What 引导主语从句时在句时在从句中充当句子成分，如主语．宾语．表语，而that 则不然。例如：

1) What you said yesterday is right. 2) That she is still alive is a consolation. 关于英语那些你不知道的事都在这里

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