1. There is the mysterious something.

something 是复合不定代词。其特征是，形容词作定语须后置。例如：

There is something mysterious.

但有时候，为了强调 something，需要特指，就可以在前面加 the 或指示代词 this， that。这时，something 就具备了名词的用法，前面加了冠词或指示代词，形容词也要前置：the/this mysterious something。由于这种用法比较少见，一般词典并没有把something 列为名词。

网友的句子便是。又如：

To this something still unseen they gave the name radium. 

他们给这种人们尚未看到的东西命名为镭。

2. Reality is something that you can't change.

something 作先行词，关系代词用that 或 which 均可，前者较普通，后者也常见。例如：

1. Language is something which fundamentally distinguishes humans from animals.  

语言是将人类与动物区分开来的基本特征。

来自柯林斯例句

2. This is something which you might have forgotten.  

这事儿你可能已经忘记了。

来自柯林斯例句

3. Toleration, moreover , is something which is won , not granted.  

容忍, 是从艰苦中争取来的, 不是别人赏赐的。

来自《简明英汉词典》

4. I see my degree as something which will train my mind and improve my chances of getting a job.  

我把学位视为可以锻炼心智、增加就业机会的东西。

来自柯林斯例句

5. They also found something which the X - ray plate did not show: a small wax figure of the god Duamutef.  

他们还发现了 X 光片子上所未曾显示的东西 —— 一只鸠阿姆特神的蜡质小塑象。

来自《用法词典》

6. The drummer was feeling the shadow of something which was coming. 

推销员感到了要发生什么事的阴影。

来自英汉文学 - 嘉莉妹妹

7. This was something which had never for a moment entered her head. 

她的头脑里从未想到过会有这种事情。

来自英汉文学 - 嘉莉妹妹

8. It was humiliating that he did so ill something which seemed contemptible.  

让他做这些在他看来是无聊的事儿,而且他还干得很不好,这真是莫大耻辱。

来自辞典例句

9. Mr. Wyburd approached something which might be giving him trouble. 

威勃德先生提起一件可能给他带来麻烦的事情。

来自辞典例句

10. Sister Manhood had salvaged something which looked like dependability.  

曼胡德护士已经捞到了一些似乎很可靠的证据。

来自辞典例句