*For general ability in the past we can use was able to though could is more common.
e.g. I could/was able to swim when I was five years old.
*For a specific event showing success after trying, we use was/were able to, managed to, succeeded in, but not could.
e.g. I was able to/managed to solve her problems for her.
I wouldn’t use could in sentence three.
- For Teachers