to anchita (and cross threaded welcome to Krzysztof,who found the volume of new words difficult)
Welcome, please don't get scared away.
On the original topic: why should operable and inoperable be opposites if flammable and inflammable mean the same.

A Spanish friend once described English as "messy". There ain't a rule we don't break, we have smooshed together two (or possibly more) grammatical systems and words can shift from noun to verb at the drop of a hat. And we make up new words whenever necessary,convenient or just when we get bored with the ones we have.

Oh well if English was neat and rule-abiding it wouldn't be nearly as much fun.