At work,I must work hard

At work,I must work hard I must work hard 是什么意思 hard at work school I(? I work hard at school. work hard at hard We must ( )(work)hard. You must work hard I am w_____ in Chinese.I must work hard at it. I'm not good at G_______,so i must work hard. i must work hard on english.短文改错,为什么要把ON改成AT? work hard on和work hard at work hard in还是work hard at?