名词性从句在英语语法中充当名词的角色，可以作为主语、宾语、表语或同位语。名词性从句可以分为四种：宾语从句、主语从句、表语从句和同位语从句，通常由连接词引导。名词性从句的语序通常是陈述语序，即主语在前，谓语在后。在使用名词性从句时，需要注意的是，连接词不能省略，特定的连接词在特定的情况下可以省略，而某些连接词在从句中不能再出现。
什么是名词性从句？
名词性从句是指在一个句子中充当名词的从句。它可以在句子中作主语、宾语、表语或同位语。名词性从句一般由连接词引导，如that, whether, if, who, whom, whose, what, which等。
名词性从句的种类
名词性从句可以分为四种：宾语从句、主语从句、表语从句和同位语从句。
宾语从句
宾语从句在一个句子中充当宾语的作用。例如：
- I know [that you are coming].
- Can you tell me [what time it is]?
- She asked me [if I had finished my homework].
连接词：that, if, whether, what, who, whom, whose, how, when, where, why, whoever, whichever, whatever等。
主语从句
主语从句在一个句子中充当主语的作用。例如：
- [What he said] is true.
- [Whether we will go] depends on the weather.
- [That he has quit his job] surprised me.
连接词：that, whether, if, what, who, whom, whose, how, when, where, why, whoever, whichever, whatever等。
表语从句
表语从句在一个句子中充当表语的作用。例如：
- My biggest fear is [that I will fail the exam].
- The problem is [how to solve it].
- The fact is [that he is wrong].
连接词：that, whether, if, what, whichever等。
同位语从句
同位语从句在一个句子中充当同位语的作用，用来对前面的名词或代词进行解释或说明。例如：
- The news [that he won the Nobel Prize] was exciting.
- Her belief [that all people are equal] is admirable.
- He announced [that he is running for president].
连接词：that, whether, if, what, whichever等。
名词性从句的语序
名词性从句的语序一般是陈述语序，即主语在前，谓语在后。例如：
- I know [that you are coming].
- [What he said] is true.
- The news [that he won the Nobel Prize] was exciting.
名词性从句的注意事项
- 名词性从句的连接词不能省略。
- 当连接词为that时，一般可以省略。
- 当连接词为if或whether时，是否省略要根据具体情况而定。
- 当连接词为who, whom, whose, which, that时，从句中不能再出现这些词。
- 当连接词为what, where, when, why, how等疑问词时，从句中一般不再出现这些疑问词。
