One of the things I always believe is that no matter how bad something is, you can take something positive out of it．The one time I wasn't sure of that was on September 11．

I usually wake up at 6∶30 or 7∶00 in the morning．That day, I happened to wake up earlier．I turned on the TV and I saw that a plane had crashed into the North Tower of the World Trade Center．My first reaction was that it was a terrible accident．Then I saw another plane fly into the South Tower and I realized that I was witnessing an act of terrorism．

My wife and I spent the day watching the terrible pictures over and over again．When I Watched family members looking for their loved ones, the pain in their eyes was something I would never forget．I couldn't understand how something this terrible could have happened．

老师好！阅读上述文字，我遇到几个问题：

一、第一段的句子the one time I wasn't sure of that was on September 11

1.出现在主语部位的the one time在资料里我没能查到用法，是否是“这一次、这一回”的意思？等同于“this time”、“the date”吗？

2.I wasn't sure of that是否是定语从句修饰先行词the one time? 定语从句的关系词是否是"when"?

3. 表语on September 11可不可以省略介词on? 原句能否写为:the one time I wasn't sure of that was September 11.

二、第三段的句子I couldn't understand how something this terrible could have happened.

烦请老师指点could have happened在这里的使用现象和含义

谢谢老师指导！