Hello same kwok, 'at the bank' It could refer to the location or to the place of action. When you are 'at the bank' you could be heading towards it , in its direction, right outside its building or inside it. It depends on the context, but in all cases it implies that the action is happening at this place. "Where did you meet him?" "At the bank" "I changed money at the bank" (The action took place at the bank). 'in the bank' means inside and within the bank. Here it indicates the location or the events inside the boundaries of this location. " I saw him walking in the bank." ( The location is inside the bank) "There is a strict code of conduct among the employees in the bank" It refers to happenings inside the bank. * The difference between 'at a bank' and 'at the bank' is that 'at a bank' is indefinite( could be any bank) ,while 'at the bank' is definite and refers to a certain bank in particular.