he thought 是插入语，不是 where 从句的主句，因为去掉后不影响定语从句的意义。

He put the money where (he thought) his wife was sure to find. 

【翻译】他把钱放在（他认为）妻子肯定能找到的地方。

有时，定语从句的关系词与从句中的句子成分之间，插入类似：

I think/believe/guess/expect 以及I am sure,they say，we know 等插入语，这种定语从句被称为“混杂式定语从句”。

Jocelyn is the only person who we expect will win. 乔斯林是我们期望唯一能胜的人。

Many college students have mastered computer, driving and foreign languages,which we all know are the most important things in today's society.

很多大学生都掌握了计算机、驾驶和外语，我们都知道这些在当今社会是非常重要的。

按逻辑判断，应该是修饰 find，修饰 thought 的话，情景是这样的：他在一个地方想“妻子肯定能找到的”，然后把这个东西放在了想这件事的地方……很明显不对。

He put the money where he thought his wife was sure to find it.

=He put the money in the place where (he thought) his wife was sure to find it.

参考译文(1)：他把钱放在他认为他妻子一定能找到的地方。

参考译文(2)：他把钱放在他认为他妻子一定能找到的地方。

这里的 he thought 到底是插入语还是 where 从句的主句？

解答如下：

(1)赞同刘老师把“he thought”分析为“插入语”成分

He put the money where his wife was sure to find it（地点状语从句）

参考译文(1)：他把钱放在他妻子能找到的地方。

He put the money where he thought his wife was sure to find it.

参考译文(1)：他把钱放在他认为他妻子能找到的地方。

(2)赞同曹老师应该要把find后面加一个“代词it”

He put the money where he thought his wife was sure to find it.

备注：插入语主要用于强调某人对某事件持有某种态度和看法。