ارجو من الاساتذة الكبار توضيح الفرق بين can and could فى حالة possibility

[elbhy]

There are two types of possibility. We can use can or could to say that "things are possible in general: that people are able to do them, the situation makes them possible, or there is nothing to stop them." (Swan p320)

These plants can grow in any type of soil.
In the past, everybody could ride a horse.

We can also talk about a degree of certainty. "To talk about the chance (possibility) that something will happen, or is happening, we use may, might or could, but not can." (Swan p 320)
Where's Mona? ~ She may be in class. (NOT She can be …)
We may go to Luxor next summer. (NOT We can go …)
"Might and could suggest a less strong possibility." (Swan p320)
It might/could snow today, but I think it probably won't. (NOT It can rain …)

Can is rarely used in the positive to express a degree of certainty, and only then if it is qualified. Do not use can in this way on an exam. We use could (and other modals) to express a degree of certainty. It is possible to use can in the negative to say that it is impossible for something to be true.
That can't be John at the door. He's travelling today.
(Michael Swan, Practical English Usage, 3rd ed.)