Can:
- To show ability (Ex.: Superman can travel at the speed of light.)
- To ask for permission (Ex.: Can I go to the bathroom?)
- To express doubt (Ex.: He cannot play on this level.)
- To show astonishment (Ex.: I cannot believe what I just saw!)
Could:
- Past abilities (Ex.: Michael Jordan could do amazing things on the basketball court.)
- Past possibilities for something that will never be known (Ex.: Could the athletes of the past have beaten the current athletes?)