bagatelo

No -el- suffix. Esperanto has -et- for small, -er- for a particle of something and -ec- for a quality, and after a few weeks with those a learner starts reading every two-syllable tail as an affix. -El- is not one of them. Bagatel- came in whole from French and Italian, and the -ella on the end is an Italian diminutive that stopped being a working part long before Esperanto existed. Modelo, hotelo, duelo and kastelo are all in the same position: one root each, nothing to unpick.

Which is a shame, because the meaning really is a diminutive. Whatever the disputed source - a small berry or a small bundle - the word was built as a little-thing word, and it kept the sense exactly. A bagatelo is something too small to be worth the attention it is getting: a trivial expense, a petty argument, a detail. The English bagatelle has narrowed to the game and the piece of music; Esperanto kept the wide sense that French and German still use, and it is a genuinely useful word for the ordinary business of telling somebody to calm down.

How it behaves. Bagatela is the adjective, trivial or negligible, and bagateligi is to trivialise something - to make a mountain into a molehill, which is a move that deserves a single verb. It sits naturally with nur, tia and pro: nur bagatelo, pro tia bagatelo. The opposite pole is grava, important, and the pair grava versus bagatela is one of the neater oppositions in the language for arguing about what matters. Note that Beethoven's Bagateloj keep the musical sense in Esperanto too, as a borrowed title rather than a second definition.