On her journey home, she fell asleep on the bus and missed her stop. That was when she decided to talk to her boss.

本句，可以用It 代替That。

分析：That was when she decided to talk to her boss.

这个句子，when 是“融合性关系副词”，等于the time when，引导表语从句。

意思是：“就在那时，她决定和她老板说明此事。”

that 指代“前文描述的事件发生后的时间”。即：乘车睡着了，忘记下车。that 就是指此事发生后的这个时间。it 不仅可以指代“事物”，也可以指代“事件”，也可以指代“事件发生的时间”。所以，完全可以用it 代替that，意思不变。

主语（表时间） + be + when ... 结构的深层分析：

表时间的主语包括代词It 和That，它们用来指代过去的某个时间（即：前文叙述的事件所发生的时间）。例句：

1. That was when they fell in love. 那就是他们相爱的时候。

2. That was when I was taught to think. 就在那个时候，我学会了思考。

3. It was when anything could happen. 那个时候时什么事情都可能发生。

4. It was when my career was just starting to take off. 那正是我的事业刚刚起步的时候。

5. Last October was when we spent our honeymoon. 去年十月就是我们度蜜月的时候。

6. Thanksgiving is when people celebrate the harvest. 感恩节是人们庆祝丰收的时候。

7. 1982 was when I got married. 1882年就是我结婚的那一年。

8. I know he called me “darling” twice, and the other time was when he said good-bye. 我知道他叫过我两次“亲爱的”，另一次是在说再见的时候。

【注意】这类句子，充当主语的为“时间名词”或者指代时间的“代词”（It, That）。when 引导表语从句，是“融合性关系副词”，等于the time when。以上例句都可以在when 之前加上一个省略了的时间名词，如the time 或者相应的词语，意思保持不变。例如：

That was the time when they fell in love. 那就是他们相爱的时候。

It was the time when anything could happen. 那个时候什么事情都可能发生。

Last October was the month when we spent our honeymoon. 去年十月就是我们度蜜月的时候。

Thanksgiving is the day when people celebrate the harvest. 感恩节是人们庆祝丰收的时候。

1982 was the time (the year) when I got married.