Esperanto is a fascinating language. Created by Zamenhof in 1887, it has become a living language with a community and even native speakers!
All living languages are subject to change. Whilst Esperanto's verb system is entirely regular, it has undergone changes and even developed some less regular features.
The letter ĥ (IPA: /x/, like loch in English) has fallen out of favour in today's Esperanto. Instead, Esperantists use k (or h if k is already taken), and pronunciation has changed accordingly.
For example, teĥnologio (en: technology) has become teknologio.
Not all words have been able to change this way. ĥromo (en: chromium, the chemical element) has become kromio, to avoid confusion with krom (en: except). Krom is often used as an adjective, such as kromaj aferoj (en: excluded matters), so chromium kromo would have created ambiguity.
Another interesting example is ĥoro (English: choir). Koro (en: heart) and horo (en: hour) already exist, so this word has not changed.
Early Esperanto had two forms of the second person pronoun: vi and ci (en: you). Today, Esperantists only use vi.
Ci was not in the original language definition by Zamenhof, but Zamenhof himself used it in language translations and a few Esperanto organisations have used it. Ci is typically used when the author desires a separate formal/informal pronoun, like tu and vous in French. Vi is the formal pronoun in these situations.
Usage of ci has become very rare today.
Zamenhof's Esperanto is highly agglutinative and has a set of suffices that may be attached to any word. For example, -aĵo (en: a thing), -igi (en: to make) and -iĝi (en: to become).
However, -ujo (en: a container) was the original suffix to use with country names. For example, franco (en: a French person) gives us Francujo (en: France).
Esperantists use -io today for countries, so France is now Francio. The suffix -io is less versatile than the original suffices, because io (en: something) is a separate standalone word. -io is only used for countries.
This is further complicated by different country names having different derivations. Whilst franco is a root word that the country name is derived from, some country names are root words.
For example, usono (en: USA) is a root word, so an American is a usonano. -ano (en: member of) is a well-established suffix in its own right, so you even see uses like franciano, which means someone who lives in France but is not necessarily French.
Having two classes of country names is unusual in a language whose grammar is so regular in most areas.
Finally, participles have fallen out of favour. Instead of saying mi estis manĝinta (en: I was eating), today's Esperantists prefer mi manĝis (en: I ate).
This is a result of a contentious discussion between the 1930s and the 1960s over the precise meaning of -ata (en: present passive participle) and -ita (en: past passive participle).
Given the phrase subite lin trafis kuglo (en: suddenly a bullet hit him), this could be written two ways using participles:
Subite li estis trafita de kuglo (approximate en: suddenly he had been hit by a bullet). The justification given was that trafita shows that the action has finished.
Subite li estis trafata de kuglo (approximate en: suddenly he was being hit by a bullet). The justification here was that he was hit at this point and time, and argues that estis trafita suggest he was hit by a bullet at some earlier time.
Zamenhof mostly used -ita, and eventually the Akademio de Esperanto declared that -ita is the preferred form here.
Esperantists today use participles, but in fewer situations. It's also common to see additional context used to prevent ambiguity, such as li estis jam trafita de kuglo (en: he was already shot).
The existence of language change is strongly indicative of Esperanto being a living language today. Esperanto started from a strongly prescriptive basis and still has the Akademio de Esperanto (en: Academy of Esperanto) to define what is 'correct' Esperanto.
In spite of this start, Esperanto is subject to the same pressures as other living languages and the speakers have the final say on how to use Esperanto today.