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Suffix -able adds meaning "being able" to a word. I know that.

Prefix in- and un- mean "not" or some negative meaning. I know that.

However, when it comes to mixing of these, I am confused.

  • unbelievable
  • indispensable

In the dictionary I can find many words of these forms, let's call them in-able and un-able, whose composing rule seems just random to me.

When given a certain word body, how can I make an in-able or un-able word to mean "not being able"?

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I am unacquainted with any helpful rule, and I fear it is unknowable, although you may find this inconceivable. (Does that word mean what I think it means?) – James McLeod Jun 22 '12 at 2:45

Consider unbelievable versus its synonym incredible, and you will find what there is to be found of an answer here.

The general tendency is to use un‑ on Germanic words or any generic English term, and to use in‑ (possibly mutated; see next paragraph) for words of Latin origin. This is not hard and fast, however; there are of course exceptions.

Also, the Latinate in‑ prefix comes in other forms, like illegible, immutable, irreducible. Those also count as in‑ versions, not un‑ versions.

Similarly, ‑able is the more general ending, but -ible also frequently occurs. In fact, you will find that ‑able and ‑ance tend to go together, just as ‑ible and ‑ence tend to go together.

Here again there is an etymological explanation: whether it derived from either a Germanic word or from a first-conjugation Latin verb (so Germanic words or Latin ‑are verbs yield ‑able type endings) on the one hand, or whether it was instead from another Latin conjugation (so ‑ere verbs and such yield ‑ible type endings) on the other.

But as before, there are notable exceptions.

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I don't think there is a rule for that. Both preffixes un- and in- usually apply to adjectives, and that is what you get when you have something-able.

The only "rule" I can remember is that for some verbs, when you want to mean the reverse action, un- is what you want (undo, unlock, untie...).

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No, there is a tendency, as @tchrist said; but it is not always reliable. – Colin Fine Jun 22 '12 at 10:06

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