1. Is putting a spare tire in your car's trunk a preventive or preventative measure?
2. If a new invention is "before its time," is it not practical or not practicable?
(1) Preventive prevails, according to purists. The adjective suffix '-ive' is commonly attached to a verb, in this case 'prevent.' The longer, more learned-sounding 'preventative' supposedly arose from a preference for its imposing meter. Some call it the pretentious form of preventive. (2) Not practicable, meaning that the invention cannot be used or put into practice now. But the implication is that the idea might be usable in the future, once technology, education., etc. catch up. Something that is not practical now, however, is not likely to become so at a later date.
SOURCES: Merriam-Webster's Synonyms; The Random House Dictionary; The World Book Dictionary; The Dictionary of Confusable Words, by Laurence Urdang.
(c) Copyright 2001. The Christian Science Monitor