mallongigo

This is the word to show somebody who does not believe you about Esperanto. Take it apart: longa, long, which they learned in chapter two. Mal-, the reversing prefix, which turns it into mallonga, short. -Ig-, the make-it-so suffix, which turns that into mallongigi, to shorten. And -o, which turns the action into a thing: mallongigo, a shortening. Four pieces, all of them already owned, and the last one had nothing to teach.

English needs a Latin loan for the same idea. Abbreviation is twelve letters and tells an English speaker nothing at all unless they happen to know brevis. Mallongigo is eleven letters and tells a first-year Esperantist everything, in order, without a dictionary. That is the whole argument for the language compressed into one noun - and note that the learner does not memorise this word, they derive it, which is why the effective vocabulary of 2,500 Esperanto roots is several times 2,500 words.

The abbreviations themselves. The one you will meet most is ktp, kaj tiel plu, and so on - the Esperanto etcetera, and itself a mallongigo of three words. Then t.e. (tio estas, that is), ekz. (ekzemple), bv. (bonvolu, please), n-ro (numero) and the titles s-ro, s-ino, d-ro. The full working set, plus the rule about where the full stops go, is in orthography and punctuation.