When do we use 'could' and 'was able to' to talk
about past abilities?
If it's a positive sentence, ask yourself
if it's a general ability - which uses 'could',
or a specific achievement - which usually
uses 'was able to'.
'I could run fast when I was younger.'
'I was able to win a medal.'
But we do use 'could' for a specific event,
if we use verbs related to senses or thought processes.
'I could feel the wind in my face.'
'I could remember my coach's advice.'
If the sentence is negative, it's much easier –
we can use either.
'I couldn't do much at school the next day.'
'I wasn't able to concentrate at all!'
Negative or positive? Specific or general?
Are there any senses or thought processes?
'What could you do when you were younger?'
'What were you able to achieve?'
'What could you see or hear while you were doing it?'