Wasn't able to ~ 和 Was unable to ~ 的差別在哪裡？如果不好說明，請提供一些例句。

"Wasn't able to" means "was not able to"

And "was unable to" means un-(prefix meaning not) able to "was not able to"

Basically they mean the same thing but in my personal experience I hear "was unable to" when someone is speaking more formally or professionally. It differs for different areas and people but I feel that the use of a contractions (wasn't) make a sentence more casual because it's kind a 'lazy' or 'shortened' way of speaking. Not always of course, you can still use contractions in formal or professional situations, but I just find that it sounds better without them. :)

1Ex: Sorry, I was unable to attend the event last night.

2Ex: I wasn't able to attend the family gathering last year.

Hope this helped!