当不定代词one 没有特别限定时，定语从句修饰后面的the stories，说明the stories 都是他写的，所以：

This is one of the stories that were written by him.

【翻译】这是他写的其中一部小说。

反之，当不定代词one 被特定时，如the only one（唯一一部小说），定语从句只修饰the only one，说明后面的the stories 不是他写的，所以：

This is the only one of the stories that was written by him.

【翻译】在这些小说中，这是他写的唯一一部。

又如：

1. Tom is one of the boys who were praised by the headmaster yesterday.

汤姆是被校长表扬的男孩中的一个。

2. Tom is the only one of the boys who was praised by the headmaster yesterday.

在那些孩子中，汤姆是受到校长表扬的唯一一个。