连词从句是英语语法中常见的一种结构，指由连词引导的从句，用于补充说明主句的内容。根据连词的性质和功能，连词从句可分为以下两类：

一、名词性从句

主语从句

引导主语从句的连词包括：

- that（无实际意义，可省略）

- whether

- if

- as if/as though

例：

> Whether she will attend the meeting is still unclear.

> The problem is that we don't have enough resources.

宾语从句

引导宾语从句的连词多为that（可省略），也可用whether、if：

> I wonder what time the meeting starts.

> He asked if she could help with the project.

表语从句

引导表语从句的连词包括：

- that（无实际意义，可省略）

- whether

例：

> The news that the project was delayed surprised everyone.

> The reason for the delay is not yet known.

同位语从句

引导同位语从句的连词通常为that：

> The manager, who is very experienced, will lead the team.

> The book, which was published last year, is very popular.

二、状语从句

引导状语从句的连词包括：

时间状语从句：when（当……时）、while（当……时）、before（在……之前）

原因状语从句：because（因为）、since（既然）

条件状语从句：if（如果）、unless（除非）

记忆口诀：

* 时间：when, while, before

* 原因：because, since

* 条件：if, unless

例：

> The meeting started when the CEO arrived.

> The project will be delayed unless the problem is solved.

三、注意事项

并列连词与从属连词

- 并列连词（如and, but）连接并列结构，不引导从句；

- 从属连词（如that, because）引导从句并表明逻辑关系。2. 省略规则

- 当从句的主语与主句主语一致且从句中有be动词时，that可省略；

- 例如：

> The book that you gave me is interesting.

> I don't know whether she will call you.

通过以上分类和示例，可以系统地理解连词从句的用法。若需进一步探讨具体语法现象，可结合实际语境分析。