Every student should regularly experience the "Aha!" — when something you never understood, or something you never knew was a mystery, becomes clear.

这是一个带有插入语you never understand, 因而省略作定从主语的关系代词that的句子。特别要注意的是这个省略掉的that在完整句中的位置:

when something you never understood, or something that you never knew was a mystery, becomes clear.因为只有you never understand为插入语，放在其前面作主语的关系代词才可以被省略，而插入语的位置一定要位于定从的关系词之后。

在举一个今年深圳市高三二诊的句子供大家参考：

Bzeek, 62, has spent the last 20 years as a foster (代养) father to children he knows are going to die, ___61___ he takes them in anyway. He has cared for the sickest of the sick in the county’s foster care system. Some have died in his arms.62岁的Bzeek领养晚期病重儿童已经20有年了，虽然明知他们会不久于世，他依然不改初心。他在县里的领养中心照顾他们，有些孩子在他的臂弯里离世。

【参考解析】题目不难，填but/yet。难在前部分。这是一个带有插入语因而省略作定从主语的关系代词who的句子。特别要注意的是这个省略掉的who在完整句中的位置：Bzeek, 62, has spent the last 20 years as a foster (代养) father to children who he knows are going to die...。