Esperanto was created by L. Zamenhof in 1887, and there are now upwards of two million people with knowledge of the language.
Esperanto was designed to be easy to learn, and grammatically simple. Whether or not it succeeds for a given learner is likely to depend on their mother tongue, and familiarity with English, Romance and Germanic languages.
For example, a large proportion of the vocabulary derives from the Romance languages (as can be illustrated by looking at the animals and colours), with the rest split between English, German and Greek.
Esperanto was designed to be easy to learn, and grammatically simple. For example Esperanto does not have grammatical gender nor does it have an indefinite article. It has a single definite article, la (similar to English 'the').
Esperanto has two cases which could be considered accusative and non-accusative.
The accusative is signified by adding an n to a word.
The word for cat and dog in esperanto are kato and hundo respectively. The word 'cat' when in the accusative becomes katon. This allows an Esperantist to change the word order depending on where the emphasis should lie. La katon mordas la hundo places more emphasis on the cat than if katon was placed after the verb (as would be more usual).
Any learner of an inflected language will immediately recognise the use (and frustration) of being able to do this. It is interesting to speculate whether L. Zamenhof being a native speaker of an inflected language (Polish) couldn't bare to remove all cases for the above reason.
Intererestingly all prepositions in Esperanto use the nominative case. This is not the case for Polish or other Slavic languages.
An extract from the Esperanto version of the Little Prince.
Iam, kiam mi estis sesjara, mi vidis belegan bildon en iu libro pri la praarbaro, titolita "Travivitaj Rakontoj". Tiu bildo prezentis boaon, kiu glutas rabobeston. Jen kopio de la desegno:
Once (a long time ago), when I was six years old, in a book about primeval forests, called 'True Stories from Nature', I saw a magnificant drawing. It was a picture of a boa constrictor in the act of swallowing an animal. Here is a copy of the drawing.
See the Lords prayer in Italian for a comparison with a romance language and the Lords prayer in Polish for a comparison with a slavic language.
Patro nia, kiu estas en la ĉieloj,
sanktigata estu Via nomo.
Venu Via regno,
fariĝu Via volo,
kiel en la ĉielo, tiel ankaŭ sur la tero.
Nian panon ĉiutagan donu al ni hodiaŭ.
Kaj pardonu al ni niajn ŝuldojn,
kiel ankaŭ ni pardonas al niaj ŝuldantoj.
Kaj ne konduku nin en tenton,
sed liberigu nin de la malbono.
(Ĉar Via estas la regno kaj la potenco kaj la gloro eterne.)
Amen.
An Esperanto dictionary : lernu.net