Not only will their critical sense atrophy but also they will not be able to tell the difference between what is right and wrong.

tell 在这里的意思是"辨别、区分"，指通过观察或判断来识别事物。 the 是定冠词，用于特指某个事物或概念。 difference 是名词，意思是"差异、区别"，来自动词 differ（不同、有区别）。 tell the difference 是一个固定短语，意思是"区分、辨别出不同之处"。