"Not able to" vs. "unable to"

word-choice negation prefixes negative-prefixes

Which phrase is more suitable to convey one's inability to do something — "not able to" or "unable to"? For example,

  • not able to join the meeting
  • unable to join the meeting

I interpret the sentences in different ways.

"He was unable to join the meeting" I read as, "he" was unable to join the meeting because of scheduling conflicts, or for reasons that made him decline participation in advance.

While I read "He was not able to join the meeting" as, "he" got held up in traffic or some other unforeseen situation prevented him from joining the meeting as he had planned.

I'm not sure there's any formal validity to this.


I think, 'Unable' denotes internal cause or inability and 'not able' external constraints.

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