Esperanto has so-called free word-order. Unlike in English, Esperanto word-order is not used to distinguish between subject and object of the sentence. This is done by case (subject is in nominative, object in accusative). Instead, Esperanto word-order can be used to express information-structure of the sentence. Simplifying somewhat, Information Structure^[1] of a sentence is the way in which the sentence "packages" information. It is the partitioning of the sentence into the theme and the rheme on the one hand and contrasted and non-contrasted part on the other. Every assertion in a dialogue answers some question (spoken or unspoken). Theme is the part of the sentence that identifies that question, and rheme is the answer. In English this is done primarily by intonation, possibly by using passive voice, particles, cleft sentences, etc. In Esperanto, theme usually precedes the rheme:

Q: Kiun mordis la hundo? – Who was bit by the dog?
A: La hundo mordis la knabon. (subject – predicate – object) – The dog bit the boy.
Q: Kiu mordis la knabon? – Who bit the boy?
A: La knabon mordis la hundo. (O – P – S.) – The boy was bit by the dog.
Q: Kion faris la hundo al la knabo? – What did the dog do to the boy?
A: La hundo la knabon mordis. (P – S – O) – It was biting, what the dog did to the boy.

There are some limitation to this freedom – prepositions have to stand before their nouns, adverb has to precede the word it modifies, etc.

^[1] There is a variaty of nomenclature. Information structure is also sometimes called Information packaging, Functional Sentence Perspective or Topic-Focus Articulation. Theme is often called topic and and the rheme is often called focus or comment. To add to the confusion, topic is sometimes used to mean only contrastive theme not any theme, and focus sometimes denotes contrast in both the theme and the rheme. More information on this topic can be found in:

• Chapter 3 of Enric Vallduví's thesis
• section 3.3. of Jirka Hana's thesis - provides some historical overview, some examples
• Daniel Buring: Semantics, Intonation and Information