(not a teacher).
To talk about ABILITY in the past you can use both but 'could' expresses ability in general, i.e. at any time - (I could read when I was four) whereas 'was/were able to' refers to a particular occasion - (I was able to read her letter in spite of her terrible handwriting). If you find this distinction difficult try using 'managed to' or 'succeded in' instead of 'was/were able to'. In negative sentences both can be used with no difference when expressing ABILITY.
Hope you find this helpful.