ABILITY : CAN and COULD

ABILITY : CAN and COULD 

Can is used to express physical ability.
- Tom is strong. He can lift that heavy box

Can is frequently used with verbs of the five senses : see , hear , feel , smell , taste
- I can see Central Park from my apartment

Can is used to express possibility.
- you can buy a hammer at the hardware store. ( you can buy = it is possible for one to buy. )

Can is used to give permisson in informal situations.
a. I’m not quite ready to go , but you can leave if you are in a hurry. I’ll meet you later.

In formal situations , may rather that can is usually used to give permission
b. when you finish the test , you may leave ( compare a and b )

Negative form : cannot or can’t.
- dogs can bark , but they cannot / can’t talk T

The past form of can meaning “ ability “ is could
- Tom could lift the box, but I couldn’t

Negative : could not or couldn’t