Mr. A first day in the office

Mr. A's first day in the office. Mr. knows everything about the job, but he wanted to pretend as if he don't know much so that he would not be asked to do over time work. But his thought went in a wrong way when his Boss dismissed him. So he went back home in a distress and sat by his mother. He recited all the flashback to her and mother advised him that he were not being careful. He should think twice before he does anything.

Please correct my sentences.