This article is from the Esperanto FAQ, by Mike Urban email@example.com and Yves Bellefeuille firstname.lastname@example.org with numerous contributions by others.
Even more than its vocabulary, it is Esperanto's grammar and rules which
makes it exceptionally easy. Unnecessary complications have been
eliminated: there is no grammatical gender, the word order is relatively
free, etc. The rules have also been simplified as much as possible:
there is only one verb conjugation, all plurals are formed the same way,
a prefix can be added to any word to change it to its opposite
(good/bad, rich/poor, right/wrong), and so on. Thus, after perhaps 30
minutes' study, one can conjugate any verb in any tense. This is a
tremendous simplification compared to national languages.
Esperanto's flexible word-order allows speakers from different language
families to use the structures with which they are most familiar and
still speak perfectly intelligible and grammatically correct Esperanto.
This also makes Esperanto an excellent translator of such different
languages as Chinese, Japanese, Latin, English and French.