'Could' and 'was able' used for past ability.
1- For ability only, either can be used (we can use 'could' or 'was able'):
'When I was young I COULD / WAS ABLE to climb any tree in the forest.'
2- For ability + a particular action (or situation), use 'was able':
'He was badly hurt but he WAS ABLE to explain what had happened.'
This rule, however, is relaxed in the negative when the action did not take place, and with verbs of the senses:
'He read the message but he COULDN'T / WAS NOT ABLE to understand it.'
'He COULD / WAS ABLE to see him through the window.'