Merriam-Webster says this:

"When cooking over a gas stove, avoid wearing loose, (flammable/inflammable) clothing that could catch fire easily." Which word is correct: flammable or inflammable?

It's a trick question. Both flammable and inflammable are correct, as they both mean "capable of being easily ignited and of burning quickly." This makes no sense to the Modern English speaker. In English, we think of in- as a prefix that means "not": inactive means "not active," inconclusive means "not conclusive," inconsiderate means "not considerate." Therefore, inflammable should mean "not flammable."

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What do you do? 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.

Personally I use "inflammable", but if I were writing warning posters I would have to reconsider that.

As a non-native English speaker I sometimes struggle with those words added after verbs: off, on, up, down, etc.

I went off my head reading this

Yes.

Because modern English is a combination of ancient Celtic languages with the various languages of the British Isles's numerous populators and conquering forces across history, including but not limited to: Latin, German, Norse, and French. And spelling wasn't really standardized until the printing press was invented, due in part to class stratification. But that's another story lol. Suffice it to say, it's more than a bit of a hodgepodge and you just gotta take it for what it is.

Romantic words were incorporated differently into other languages as the Roman Empire's influence spread. Words like inflammable have more direct Latin roots than words like indebt. So where Inflammable comes directy from the latin inflammo, indebt came from old French and middle English, going from something like endetter and endetten, using the root word dete and dette, but stemming from the root Latin word, debitum. Then en became in at some point.

That's how I understood it anyway through a very short amount of google searching. Anyone feel free to correct me.

Where the words come from. Remember: English is many languages in a trench coat, not really its own set of hard rules.

Hell, even languages with hard rules like Japanese have some exceptions even without the modern trend of borrowing words from other languages!

Also in Italian: infiammabile