Table of correlatives (Esperanto)

The word base of Esperanto was originally defined by Lingvo internacia, published by L. L. Zamenhof in 1887. It contained some 900 root words. The rules of the language allow speakers to borrow words as needed, recommending only that they look for the most international words, and that they borrow one basic word and derive others from it, rather than borrowing many words with related meanings. In 1894, Zamenhof published the first Esperanto dictionary, Universala vortaro, which was written in five languages and supplied a larger set of root words.

Since then, many words have been borrowed from other languages, primarily those of Western Europe. In recent decades, most of the new borrowings or coinages have been technical or scientific terms; terms in everyday use are more likely to be derived from existing words (for example komputilo [a computer], from komputi [to compute]), or extending them to cover new meanings (for example muso [a mouse], now also signifies a computer input device, as in English). There are frequent debates among Esperanto speakers about whether a particular borrowing is justified or whether the need can be met by derivation or extending the meaning of existing words.

Esperanto occupies a middle ground between "naturalistic" constructed languages such as Interlingua, which take words en masse from their source languages with little internal derivation, and a priori conlangs such as Solresol, in which the words have no historical connection to other languages. In Esperanto, root words are borrowed and retain much of the form of their source language, whether the phonetic form (eks- from ex-) or orthographic form (teamo from team). However, each root can then form dozens of derivations that may bear little resemblance to equivalent words in the source languages, such as registaro (government), which is derived from the Latinate root reg (to rule).

One of the ways Zamenhof made Esperanto easier to learn than ethnic languages was by creating a regular and highly productive derivational morphology. Through the judicious use of lexical affixes (prefixes suffixes), the core vocabulary needed for communication was greatly reduced, making Esperanto a more agglutinative language than most European languages. It has been estimated that on average one root in Esperanto is the communicative equivalent of ten words in English.

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