Think about the eight letters A B C D E F G H, if A and B (or any two letters) are already chosen then you want to know how many ways 6 letters can fill 3 spaces when order matters, in other words the letters F G H are considered 1 outcome with the first two letters and G F H is considered another individual outcome with the first two letters. If order didn't matter then F G H and G F H with the first two letters would be considered only one outcome.
