I must work hard at English or I must work hard to know English better or to master English

It's possible to say 'work hard for English' in this context, especially if you mean an English exam or qualification. You can also say 'in English' here, meaning in English lessons or in the English part of your curriculum. But I'd agree with Mariya that the best preposition is 'at'. To 'work hard at' is a standard expression meaning to make an effort to improve in a particular area.