we know that "you may know that..." is used in the present or "you may have known that..." is used in the past?
But when you say "you may know that..." to a listener, then it could be that that listener may already have known that in the past.
For example,
A: You may know that your mom will never do that
B: Right (but there is a high chance that B may have known that a few days ago.
So when to use "you may know that..." and when to use "you may have known that..."?
If you use "You may know that..." to a listener, you are implying that they know it now, and they may have known it for some time, but the relevance of the knowledge is right now.
If you use "You may have known that..." the relevance of the knowledge is in that past time -- and presumably if they knew it then, they still know it now.
So a case in point would be "While you were studying at the University you may have known that Alison was there as well." You may know now that Alison was there, but did you know at the time? Because if you had known, you might have made efforts to make her acquaintance, whereas if you have only recently become aware that she was a fellow-student, acquaintanceship would not have been an issue!
