What is diffrence between could and can?

I want to know that What is difference between could and can? Where we can use it? thanx

Can - Present and Future tense.
Could - The ability / Past tense / politeness.

I could be here at 5 pm = I came to here at 5 pm - This person telling the ability/possibility of coming at that time (Past tense)
Could you lend me your pen = politeness.
I can come = The ability of the person to come (maybe in the future/present).

Can is more certain than could and in some context could is used to express something in a polite way e.g could you show me the way to…, please ?