"Forget to buy something" 可以用英语表示为 "forget to purchase something" 或者 "neglect to buy something".

这个表达的来源是英语中 "forget" 和 "buy" 这两个单词的结合。"Forget" 是动词，意思是 "忘记"，"buy" 是动词，意思是 "买"。这两个单词结合起来就形成了 "forget to buy" 这个短语，意思就是 "忘记买"。

下面是一些使用这个短语的例句：

I forgot to buy milk at the grocery store. (我在杂货店忘记买牛奶了。)
I neglected to buy a gift for my friend's birthday. (我忘记买朋友生日礼物了。)
I completely forgot to buy bread on the way home. (我回家的路上完全忘记买面包了。)