怎么就一下能看出一个从句是名词性从句，怎么知道缺不缺成份？

把从句找出来之后，假设从句不在了，看主句缺什么成分，缺主语的话，就是主语从句。例如下面的主语从句：
1. Who was responsible for the accident is not yet clear.
2. It’s a pity that you can’t go with us.
3. What he did is not yet known.
4. It is said that he has been there many times.
5. It is not yet clear who was responsible for the accident.
6. When they will have the sports meet is still a question.
如果去掉从句，主句中缺了宾语，那么这个从句就是宾语从句。例如：
1. Do you know whose mobile phone was lost? 你知道谁的手机丢了吗？
2. We are looking into what caused the accident. 我们正在调查造成事故的原因。
3. People should pay more attention to how we can stop pollution. 人们应更多的注意控制污染的方法。
4. Could you tell me where you are from? 请问你是哪儿来的？
5. Has she informed you when they are to hold the meeting?
6. I agree with whatever you have said. 你说的任何事我都同意。
同样的道理表语从句：
1. The problem is who we can get to replace her.
问题是我们能找到谁去替换她呢。
2. The question is how he did it.
问题是他是如何做此事的。
3. This is what I want,
这就是我想要的。
4. The reason why he was late is that his bike broke down.
他迟到是因为自行车坏了。
同位语从句放在某名词后，对其进一步解释，说明该名词的具体内容。
常见引导词：that, when, where, whether, why, who, whom, whose, how…
同位语从句常放在下列名词后: news, word, idea, fact, truth, reason, thought, doubt, belief, hope, promise, possibility, order, proposal, advice, suggestion, demand…
There was little hope that they would survive.
Word came that our team had won.
I have no idea why she left.
They had the question whether he could pass the final exam.

名词性从句里缺不缺成分也是这个道理。