Slang——very informal, sometimes offensive, language that is used especially by people who belong to a particular group, such as young people or criminals。
Idiom——a group of words that has a special meaning that is different from the ordinary meaning of each separate word. For example, 'under the weather' is an idiom meaning 'ill'。
而俗语却可以大胆使用，因为它们其实就是一些约定俗成的搭配用法，被社会各个阶层所使用，比如 “has two left feet”，字面意思看不出来什么意思，实际上指的是动作不协调，小脑不发达之类的。
“我最近在准备people这个系列，我想请问话题PERSON who looked after you，我把look after一两句带过，然后就讲这个人对我的影响什么的可否?那么PERSON you want to be similar to,HELPFUL person,the person you admired,the person influent you以及老人的话题是否可以都借用同样一个内容了?”