Can, Cannot, Could, Could not

Can

I can play guitar.

I can bake biscuits.

You can swim very well.

Use of CAN in the above sentences express ability in the present.

I cannot write with my right hand.

She cannot come early.

You cannot speak correct English.

Could

I could run 5 miles daily when I was in the army.

I could sing well in my school days.

He could speak Punjabi fluently as a child.

The use of COULD expresses ability in the past.

She could not score above 50% in school.

I could not even one mile a day.

He could not write one correct sentence.