silabo

This is the word the rest of the pronunciation rules were waiting for. Esperanto stress is fixed: it falls on the penultimate syllable of every word, always, with no list of exceptions and no dictionary to consult. A-MI-ko, ta-BLO, es-pe-RAN-to. Say that rule without the word silabo and you have to paraphrase it every time; with the word, it is one sentence. Akcento is the other half of the pair, and between them you can state the whole of Esperanto prosody.

Counting them is easy here, and that is not true everywhere. One syllable per vowel letter, and Esperanto has five vowels that never change value. The only trap is the two diphthongs aŭ and eŭ: ŭ is a semivowel and does not start a syllable of its own, so aŭ-to is two syllables and not three, and Eŭ-ro-po is three. Everything else is arithmetic. Compare English, where fire, hour and every start arguments among native speakers.

Where it earns its keep is verse. Because stress is fixed on the penultimate syllable, every unelided Esperanto word ends unstressed, so poetry can only rhyme feminine unless the poet does something about it - and the something is the Rule 16 elision that turns koro into kor' and buys back a syllable and a stressed line-ending at the same time. Poetic Esperanto works through it properly, including Zamenhof's single elision in the first stanza of La Espero, which is there for no reason other than keeping a ten-syllable line at ten. The adjective is silaba, and unusilaba and plursilaba decode on sight.