核心规则
表语从句位于 be、seem、look 等系动词之后，对主语进行解释或说明。常用连接词包括 that、whether、what、who、why、how、where、when、because 和 as if/as though。
结构与例句
The fact is that we need more evidence。The question is whether the change is practical。This is what we discovered。That is why the meeting was delayed。It looks as if it is going to rain。
使用场景
表语从句特别适合组织“问题—答案”“原因—结果”和“事实—内容”。连接词要准确对应逻辑：The reason is that...；This is because...；This is why...。
常见错误
错误： The reason is because the road was closed. → 标准表达常写 The reason is that the road was closed。
错误： The question is if we should continue. → 正式表语从句通常使用 whether。
易混知识点
because 引出原因，why 引出结果：The road was closed. That is why we were late；We were late. That is because the road was closed。
总结
先识别主语需要补充的是内容、选择、原因还是结果，再选择对应连接词。
选择性必修 · 表语从句的逻辑连接
高中学习重点
as if/as though 引导的表语从句可陈述看起来真实的情况，也可表达与事实距离较大的假设。若说话人认为可能真实，使用正常时态；若明显与现在事实相反，常用一般过去时：He looks as if he knew the answer。
主语为 advice、suggestion、request 等表示建议或要求的名词时，表语从句可使用 (should) + 动词原形：My suggestion is that we (should) begin early。
小练习
- 填空：The reason for the delay is ___ the road was blocked.
- 选择：The question is if / whether we can finish on time.
- 填空：My advice is that you ___ (keep) a learning journal.
参考答案
that。whether。(should) keep。