If you want to keep things crystal clear, choose flammable when you are referring to something that catches fire and burns easily, and use the relatively recent nonflammable when referring to something that doesn’t catch fire and burn easily. Inflammable is just likely to enflame confusion.
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It makes more sense if you think of it as enflammable. Indent and indebted at examples of this “in-” prefix. https://www.merriam-webster.com/grammar/flammable-or-inflammable
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