vokalo

A metalanguage word, and the course needs it early. Vokalo is not vocabulary about the world; it is vocabulary for talking about the language itself, alongside litero, silabo, akcento and akuzativo. Being able to say la akcento falas sur la antaŭlastan silabon in Esperanto rather than in English is the point at which a learner stops translating grammar and starts holding it, and this is one of the five or six words that unlocks it.

Five of them, one sound each. A, e, i, o, u, and every one keeps the same value in every word, stressed or unstressed, which is the promise the alphabet makes and keeps. English has five vowel letters and something like twenty vowel sounds, so a learner coming from it is carrying a habit that has to be put down: e in nepre is the same e as in sed, and o never turns into the neutral mumble English gives it in the last syllable of nation. Ŭ is not a sixth vowel - it is a semivowel and only ever appears after a or e, in aŭ and eŭ.

Where it earns its keep. Counting vowels is how you count syllables, and counting syllables is how Esperanto verse works, so the word turns up on every page of poetic Esperanto. The apostrophe elision of Rule 16 exists precisely to delete an unwanted final vowel and drop a beat: de l' mondo for de la mondo. And the reason vokalo is not built from voki, to call, is that the two only meet a long way back in Latin - vox is the voice, vocare is to call it out, and by the time either reached Esperanto they were separate roots.