something

这句话在说 you should buy something. 不理解你就死记住。

something

按理说疑否句应该用anything啊

两者的意思有差别。
Why didn't you buy anything for yourself? 你为啥没有买任何东西给自己？
Why didn't you buy something for yourself? 你为啥不买点东西给自己？

@夜雨寒楼
在选择题里，选 anything 是最安全的。但我试过 google 一下 why didn't you * something 也存在的（虽然比 anything 得出的结果少许多）。
我查了一下 Cambridge Grammar of English，有以下的解释，但是楼主的题目里用 why 开头，我就不懂了
"Something" and "anything" can be used to ask negative questions. They contrast in meaning:
Didn't she contribute something to the appeal? (suggests that she probably did)
Didn't she contribute anything to the appeal? (indicates greater uncertainty)
这跟 some 和 any 的解释相像：
Negative interrogatives with "some" and "any" are used to check the speaker's understanding of a situation.
Negative interrogatives with "some" typically indicate that the speakers wish to confirm their positive expectations or assumptions about the situation:
Wasn't there some trouble in the village last night? (speaker has reason to think there was, and seeks confirmation)

语气表情求询问的用肯定语气something