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  1. He cannot reach there
  1. He is not able to reach there

I think :-

  1. talks about present ability,

  2. about permanent disability that doesn't allow him to reach there.

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    You haven't given enough context for a definitive answer. For example, in some contexts, He cannot do X might mean He must not be allowed to do X, but He is not able to do X would never be used with that sense. Words only have meanings in context - and sometimes that implies a lot of context. But I'm not convinced that the can / be able to choice ever has any special implications regarding ephemeral / permanent states. Jul 5, 2021 at 12:30
  • ...pointing at a friend's old school class group photo, you might say That can't be you in the front row! Surely you were never that fat! - where it's quite natural to use can't to express "surprise" even if the surprising fact is manifestly true. But [not] be able can't normally be used like that. Jul 5, 2021 at 12:36
  • @FumbleFingers In formal written English, it would be more correct to say “may not” when you mean “is not allowed to,” but “cannot” is, as you say, often used to mean that as well.
    – Davislor
    Jul 5, 2021 at 20:00

2 Answers 2

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According to some textbooks, they mean exactly the same thing. However, “cannot” is sometimes a synonym for “may not” or “is not allowed to,” so the first sentence might also mean something else.

There is no distinction between whether someone is temporarily or permanently unable to do something. Either phrasing can be used in both of those situations.

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I think they can mean many things without a defined context, it may be that in the first sentence he must not be allowed to reach that, as a prohibition. in the other sentence, it may be that he is not able to do that at that moment, but maybe time later he does.

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