请问这是个什么从句?

Question

Sam was sure that the wallet must have been found by one of the villagers,but it wasn't returned to him.
为什么是宾从?sure不是个形容词吗?

Answer 1

一、这是并列复合句

第一分句：Sam was sure that the wallet must have been found by one of the villagers. 其中：
主句：Sam was sure
that 引导的从句：that the wallet must have been found by one of the villagers,

第二分句：but it wasn't returned to him.

二、“be + 形容词”构成的系表结构的复合谓语，其后可以带有 that 引导的从句。这种从句的语法成分在目前有几种权威说法：

一说是原因状语，即说明表语形容词产生的原因（薄冰）；
一说是形容词补足语，即进一步说明表语形容词这种状态的具体含义（李基安）；
一说是宾语，即把系表结构视作一个及物动词，后面带的是宾语从句。无论哪种说法，都有一定的道理，初学时不必较劲，明白道理足矣（张道真）。

举例如下；

I am afraid (that) I'll be late. 我恐怕要迟到了。
He was very sorry (that) you had not written to him for long. 他对用很久没有给她写信深表遗憾。
I am sure (that) he is right. 我肯定他是对的。

Answer 2

宾语从句
that the wallet must have been found by one of the villagers

Answer 3

宾从

Answer 4

宾语从句

Answer 5

宾语从句
Sam was sure 是主谓宾格式，that the wallet must have been found by one of the villagers是对宾语的补充说明。