How differentiate between -able and -ed adjectives?

I really need your help regarding this matter, In spite of all the hard work in finding a clear difference that makes me use -able over -ed, I couldn’t find a definite rule.

For example, valued vs valuable, reputed vs reputable, desirable vs desired

is there such a simple way to differentiate and know which one to use?

Answer

You cannot find any reference to such a rule because there is no such rule.

In general, the past participle of any transitive verb can be turned into an adjective. Similarly, adjectives can be formed by adding the suffix able to the root of a regular transitive verb. (An irregular verb like eat may have an irregular analog like edible.) The two adjectives thus formed usually carry the same or very similar meanings, but if the able form becomes a common word, it is likely to take on a distinct meaning.

A valued book means only that someone does or did value the book for some personal reason. A valuable book means more than that someone values the book personally: it asserts that the book has intellectual importance or substantial market value.

In the case of reputed and reputable, their meanings have become almost opposite. If I say Joe is a reputed lawyer, I am saying that others apparently believe Joe to be a lawyer but that I personally am doubtful whether he is a competent lawyer or even a lawyer at all. Reputed conveys doubt: it does not convey personal conviction. On the other hand, reputable implies that the speaker has no reason to doubt the common opinion. If I say Joe is a reputable lawyer, I am saying that as far as I know Joe is a competent and honest lawyer and deserves a good reputation as a lawyer.

Just because two words derive from the same root does not make them synonyms.

DELAYED EDIT: A comment by the OP pointed out that my original answer may be misinterpreted. When I said that able can be added as a suffix to regular transitive verbs, I did not mean to imply that such a suffix can never be added to an intransitive verb. I am not aware of any regular transitive verb where able cannot be added as a suffix. If there are any such verbs, they must be rare. On the other hand, I am aware of common intransitive verbs that cannot grammatically be changed that way. Comeable, beable, becomeable are not idiomatic expressions.

Attribution
Source : Link , Question Author : Ghassan Saeed , Answer Author : Jeff Morrow

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