Why aren't Croatian "plam" (meaning "flame") and English "flame" considered to be related, or at least possibly related? They mean exactly the same, and they seem to fit phonologically. Croatian 'p' regularly corresponds to English 'f' by the Grimm's Law. So, why is the English word being a borrowing from Latin "flamma" and Croatian word being related to "pepeo" (ash) considered a more likely explanation?
English flame isn't a native English word, but a loan from Old French flame; Grimm's law doesn't apply, as it had long ceased to operate at that point. The initial f actually reflects PIE *bʰ, which would yield b in both Slavic and Germanic but became f in Latin.
Both English and Serbo-Croatian do have native descendants of the PIE root *bʰel- that yielded flame: English bale[fire] (now archaic), Croatian bijȇl 'white'. The similarity between plȃm and flame, however, is just a coincidence.
How do we know "flame" is not a native English word dating back to Proto-Indo-European? Dec 12, 2020 at 21:43
10@FlatAssembler The word doesn't show up in Old English at all, and is only attested from the 14th century on, at a time when Middle English was in the middle of appropriating a massive amount of vocabulary from Old French in the centuries after the Norman invasion. It also doesn't exist in other Germanic languages except Dutch (which also had close contacts with French) and German (which borrowed it from Dutch; there we have the additional point that if it had been native it would have been Flahm by comparison with English and Dutch, but instead it's Flamme). Dec 12, 2020 at 22:34
Great answer! I have a doubt though: PIE *bʰ, according to all the stages of Grimm's law, should yield an f, correct? (bʰ > b > p > f). So, why did english have "bale" instead of something like "fale"?– QwertuyDec 13, 2020 at 11:53
5@Qwertuy That's how the chain shift is usually represented, but it's only applied once: PIE *bʰ changed to *b, but it didn't then keep shifting after that. Likewise, PIE *b became *p (e.g. *h₂ébōl > *aplaz 'apple') but didn't then keep moving into *f. Proto-Germanic wasn't all voiceless fricatives. Dec 13, 2020 at 13:04