But you should only use ‘was/were able to’ with action verbs to talk about an ability related to a single event or incident in the past.
I was able to surprise my teacher yesterday.
Don’t say: I could surprise my teacher yesterday.
This is an excerpt from a site. (https://blogs.transparent.com/english/cancould-vs-to-be-able-to/)
I can't understand what that means.
Why can't "could" be used with action verbs in the past tense?