human: Discuss the history and development of the Esperanto language, including its grammar, vocabulary, and international usage today.

assistant: __History and Development of Esperanto__

Esperanto is an artificial language, developed in the late 19th century by Dr. Ludwik Lejzer Zamenhof. 

Zamenhof, who was a Polish-Jewish ophthalmologist, published his first book, __"Unua Libro"__ (First Book), on the subject in 1887. He did this under the pseudonym __Dr. Esperanto__, which means "one who hopes" in the language itself. He intended Esperanto to be a universal second language that would foster peace and international understanding.

Here are some of the key events in the history and development of Esperanto:

- 1887: L. L. Zamenhof publishes Unua Libro, the first book describing Esperanto.
- 1905: The Fundamento de Esperanto, the basic text of the language, is published.
- 1921: The Universal Esperanto Association is founded.
- 1954: Esperanto is recognized by UNESCO as an auxiliary language.
- 1989: The first World Esperanto Youth Congress is held.
- 2005: The 120th anniversary of Esperanto is celebrated.

__Grammar and Vocabulary__

The grammar is designed to be simple, regular, and easy to learn. Its structure draws on elements from various European languages. Some key features include:

- Nouns end in -o and form plurals by adding -j (e.g., hundo - dog, hundoj - dogs; libro - book, libroj - books; domo - house, domoj - houses).
- Adjectives end in -a and agree with the noun they modify in number and case (e.g., bela hundo - beautiful dog, belaj hundoj - beautiful dogs; granda libro - big book, grandaj libroj - big books; nova domo - new house, novaj domoj - new houses).
- Verbs have only one form for each tense and mood, ending in -i for the infinitive, -as for the present, -is for the past, -os for the future, -us for the conditional, and -u for the imperative (e.g., paroli - to speak, mi parolas - I speak, mi parolis - I spoke, mi parolos - I will speak, mi parolus - I would speak, parolu! - speak!; lerni - to learn, vi lernas - you learn, vi lernis - you learned, vi lernos - you will learn, vi lernus - you would learn, lernu! - learn!).
- Pronouns, articles, and prepositions are also straightforward and consistent (e.g., mi - I, vi - you, li - he, ŝi - she, ĝi - it; la - the; de - of, en - in, al - to).

Esperanto has a highly regular grammar, with no irregular verbs, a feature that sets it apart from most natural languages. The language uses an agglutinative morphology, which means that words are often made up of sequences of relatively invariant morphemes.

The vocabulary is based mainly on Indo-European languages, with influences from Romance, Germanic, and Slavic languages. Around 90% of its roots come from these sources. Zamenhof aimed for the vocabulary to be neutral, avoiding words with strong cultural or national associations. 

He also devised a system of affixes to expand the language's expressiveness without needing a vast number of unique words. For example, from the root “san” (healthy), we can get “sana” (healthy), “malsana” (sick), “saniĝi” (to recover), “sanigi” (to heal), “malsanulejo” (hospital), and so on.

Esperanto's 16 rules of grammar are famously invariant, with no exceptions, which is another factor that makes the language relatively easy to learn compared to natural languages. The rules cover aspects such as word order, noun and adjective agreement, verb tenses, and the use of prepositions.

__International Usage Today__

While it's hard to get exact numbers, estimates suggest that there may be anywhere from 100,000 to 2 million Esperanto speakers worldwide as of 2021. There have been several attempts to promote Esperanto as a global lingua franca, but none have gained widespread acceptance.

Nevertheless, Esperanto has a vibrant international community. The Universal Esperanto Association organizes annual World Esperanto Congresses, which attract attendees from around the world. Moreover, the language is taught in courses and workshops in various countries and is also available on popular language-learning platforms.

Online, Esperanto has a significant presence. There are websites, blogs, podcasts, and YouTube channels dedicated to the language. It even has a presence on social media networks, and there are Esperanto editions of Wikipedia and other online services.

While Esperanto has not achieved Zamenhof's dream of becoming a universal second language, it remains the most successful constructed international auxiliary language, demonstrating enduring appeal and utility.