4  Though the crew were at first under the impression that the lost ship had been found, the contents of the chest proved them wrong.新概念3课文

结构分析：多重复合句。Though引导让步状语从句，此从句中又含有一同谓语从句that the lost ship had been found，对impression作追加说明。

under the impression that == thinking that

I am under the impression that he is the manager of the store.

He is under the impression that you'll help him.

prove +宾语+补语

One mad action is not enough to prove a man mad.

5  What they had in fact found was a ship which had been sunk many years before.

结构分析: 多重复合句。What they had in fact found主语从句。which had been sunk many years before.修饰ship。

需定语时用 sunken

The ship had sunk

a sunken ship

personal belongings 个人所属物品

together with 还有