Ability

Can is used to express general ability;the form in the past is could.

1.He can swim.

2.Jane could swim when she was 5.

To express a particular ability on one occasion in the past, could is not used. Instead, was able to or managed to is used.

1.Ben was able to go to university when he was 16.

2.The pupils managed to write from the dictation without any mistakes.