为什么这儿要用【that happens】？ 谢谢！

happen为不及物动词，一定要用所发生地事情作主语；指“某人发生某事”，要加介词to.
例：What happened to Jim yesterday?吉姆昨天发生了什么事。

When we warn children that no place is safe ,they wait for danger as if it were an event that happens.
此句中的that为定语从句的引导词，指代前文已提到的an event,并在从句中作happens的主语。

祝你进步！

sth. that happens 是事情，事件的意思但在这里没有确实意思，所以不翻译
that happens the same time 交相辉映
用法很少，只要有这两点

that happens在这做的是event的定语从句