Which one is the right one?
I am not surprised that Mike is in prison. He robbed {a|the} bank two years ago.
Which one is the right one?
I am not surprised that Mike is in prison. He robbed {a|the} bank two years ago.
It depends about which bank you are talking about.
If you know which bank and the fact that he robbed this bank in particular is important, then use "the".
If it does not matter which bank he robbed or if you do not know which bank it is, then use "a".