[误] At first, you must work hard.

[正] First, you must work hard.

注：at first 指“起初，开始的时候”，如：At first I found this question very difficult, but finally I managed to

answer it(一开始我觉得这个问题很难回答，不过最后我还是答出来了);first 才是指“首先”。又如：First, let me introduce myself(首先，请允许我介绍一下我自己)。