What on earth did you learn from the teacher last Saturday?
Why on earth would I want to work for you?
1)on earth跟在疑问词后边,是固定用法吗?可以将on earth位置变动?比如放在最后。
2)除了what、why之外,是否所有疑问词后都可以加on earth? 用以加强疑问语气。
on earth 1:... 2:—— used to make a question more forceful (用于加强疑问语气)究竟;到底 Why on earth [= in the world, ever] did you do that ? 你究竟为什么那样做?What on earth is he talking about ? 他到底在讲些什么?Where on earth are my keys ? 我的钥匙到底在哪里?(摘自《韦氏高阶英汉双解词典》p.679)