How are you doing? - A common greeting in English.
It's raining cats and dogs - An idiom meaning it's raining very heavily.
Break a leg - A way to wish someone good luck, especially before a performance.
Piece of cake - Means something is very easy to do.
Hit the books - To study hard.
How are you doing? - 这可不是问"你在干啥呢"，其实就是"最近咋样啊"的意思，跟咱北京人见面问"吃了吗"一个意思。
It's raining cats and dogs - 这可不是天上掉猫狗，翻译成"下大雨"就完事儿了，要是想说得更地道，可以说"这雨下得跟瓢泼似的"。
Break a leg - 千万别直译成"断条腿"，这是祝人好运的意思，跟咱说"加油"差不多，特别是演出前用。
Piece of cake - 小菜一碟儿！这翻译绝了，跟原意一模一样，都是说"这事儿特简单"。
Hit the books - 这可不是让你打书，意思是得"埋头苦读"了，跟咱说"啃书本"一个意思。
文化小贴士：这些俚语老外天天用，要是直译可就闹笑话了。咱翻译讲究的是传神，不是字对字。