其后的表语从句常用that引导，不可用because。

　The reason why he was late was that he missed the first bus this morning.

他迟到的原因是他今天早上没赶上早班公共汽车。

②because引导的表语从句常用于that's because...结构中，说明产生上述结果的原因；why引导的名词性从句说明由上述原因导致的某种结果。

(2)主语是表示"建议、命令、要求、计划(suggestion，advice，proposal，order，command，request，requirement，demand，plan)"等的名词时，表语从句应该用虚拟语气，即 "should＋动词原形"，should可以省略。

My suggestion is that we(should)set off early tomorrow.我的建议是我们明天早早动身。

达标检测：

1. Can you tell me _________ he will come back?

2. I want to know __________she has gone.

3. I wonder __________ he did so.

4. The reason why he is grateful for is ___________ you helped him out.