It depends on the situation.
If I have the time... (and this is something I do not yet know) .... I CAN come to see you.
So if I find I DO have the time then I definitely CAN come to see you. (can - means it is possible to do so now - and presumably I will go to see you)
If I have time tomorrow, I can book you in for a check-up. (says my doctor)
If I have 30 minutes spare tomorrow, I can chat on the phone with you. (So we wait to see if I have time)
I may/might come and see you tomorrow, if I have time..... (well, I might or might not have time/ I might go and visit you, or I might not. Even if I do have time I might not!)
I might teach the class tomorrow, if I have time. (This leaves everybody wondering about whether you are going to turn up at all!!)
I CAN teach the class tomorrow, if I have time. (This gives everyone a little more hope. Provided time is available, it's possible for me to do it. My tone of voice will tell the listener how willing I am to teach.)
If I have time tomorrow, yes, I definitely CAN teach the class tomorrow. I'll let you know later. See you then, I hope.
If I have time tomorrow, yes, I definitely might teach the class. Let me see how I feel about it tomorrow. I may have a party to go to instead. Bye. (|This is not very satisfactory from the point of view of the school principal who is trying to arrange teaching cover!)
Can you see the difference?