"困难的日子" 可以用英文表示为 "difficult days" 或 "tough times"。

关于这个表达的来源，"difficult days" 和 "tough times" 都是常用的习惯用语，表示遇到困难或艰难的时期。它们可以用来描述生活中的各种困难，比如经济困难、健康问题、情感困扰等。

下面是一些使用 "difficult days" 和 "tough times" 的英文例句，以及它们的中文翻译：

"We've been going through some tough times lately, but we'll get through it together." (最近我们遇到了一些困难，但我们会一起度过难关的。)

"She's been struggpng with depression for years, and these past few months have been especially difficult for her." (她已经患有抑郁症多年了，过去几个月对她来说特别困难。)

"It's been a difficult year for everyone, but we've made it through and come out stronger on the other side." (对于每个人来说，这是一个困难的一年，但我们度过了难关，并在另一边变得更强大。)