You are making a big mistake.
你正在犯一个严重的错误。
It's all been a ghastly mistake.
这是个极其恶劣的错误。
She paid dearly for her mistake.
她因犯错误而付出了巨大的代价。
The engagement was a bad mistake.
这婚约是一项大错。
He was loath to admit his mistake.
他不愿承认自己的错误。
I made a stupid mistake.
我犯了个愚蠢的错误。