t's a matter of time before they will discover John has left.此句(出自人教版高一下)是否有误?

问题描述:

t's a matter of time before they will discover John has left.此句(出自人教版高一下)是否有误?

不敢断言是否有误,但总感觉从句中如果掉will更通顺些.