"I speak a little Esperanto."
Translation:Mi parolas Esperanton iomete.
Esperanta is an adjective, it describes (or "modifies") nouns. For example, you can have esperanta vorto. An Esperanto word. Esperante is an adverb, it modifies verbs, adjectives and even other adverbs. In this case it was in the example because it is modifying the verb "parolas". In the same way that "speaking slowly" is "malrapide parolas", "speaking Esperanto-ly" is "esperante parolas".
There are many ways to say the same things in esperanto due to its simple structure and openness to other languages. The purpose of allowing all these different manners of saying things is so that when speaking esperanto, it hardly feels any different from speaking your mother tongue, no matter what it is (in most cases).
Have you figured it out in the four years since August 2016 when you posted your question? If not, could you elaborate what's not clear in Stephie Rice's explanation from February 2017. It's got 20 upvotes, so it seems to have helped a lot of people. Maybe it could help you too.
It has to do with whether or not its being used as an adjective or an adverb, or in the case of "Esperanto" a noun. Esperanta is used as an adjective. For example, if you were to say "an esperanto book", you would say "Esperanta libro". However, if you wanted to say "spoke in Esperanto" you would say "parolas Esperante" because Esperante basically means "in Esperanto" as an adverb. Not as an adjective, though- if you wanted to say the book previously mentioned was in Esperanto rather than about Esperanto, ypu coild specify with "libro en la Esperanta lingvo" meaning "book/a book in the Esperanto language". If you said "Esperanta libro" to sustitute for that anyways they would probably understand, they just moght confuse it for a book about esperanto rather than in esperanto, but since "in Esperanto" is not being used as an adverb you wouldn't say Esperante.
It would be understood if you said it this way even though it is not very natural and it could be interpreted ambiguously.
More natural would be "Mi parolas iom da esperanto". The word "iom" is used to signify a quantity of something and as such it is used often to describe nouns. This could still be interpreted strangely.
I prefer to use "iomete" in these situations so that it isn't assumed I am a fluent speaker who just happens to be speaking only a select amount.