零基础计算机入门小问题

[txxuer]

刚开始学计算机，通过网上看课程。可是没有老师解答，只有答案，有不明白的地方想请教。
Here is the grammar written in a way that more clearly distinguishes nonterminals (Italics) and terminals (bold):

Something -> Word Something
Something -> Word
Word -> Hey!
Word -> Listen!

Check the box for each string that cannot be produced starting from Something.
没明白这啥意思，我总是会不自觉的从后往前推。