A good polyglot learns to take the rules of any given target language for granted as natural laws; a good linguist on the other hand learns that there are many different ways of doing things. Esperantists (who tend to be hobbyist Euro‐polyglots) often trumpet the language's case‐marking system as an indispensable guide to the fundamental “argument structure” of a sentence. But even disregarding the way Esperanto marks various other things the same way as its “direct objects”, there's nothing logically necessary about subjects and objects. Indeed, the terms are only meaningful once you've defined them for a specific language in terms of the more universal concepts of:
- Agent = “Subject” of a transitive verb (“WE saw Sam”)
- Experiencer = Argument of an intransitive verb (“WE waited”)
- Patient = “Object” of a transitive verb (“Sam saw US”)
Different languages group these according to various schemes (known to linguists as “alignments”).
the pedant's solution (known from one Australian language).
Clearly more complicated than there's any call for.
Agent distinguished as Ergative case Experiencer distinguished as Intransitive case Patient distinguished as Accusative case
the clairvoyant's option (less rare; much use of context): cases
not distinguished even by word‐order rules.
Agent / Experiencer / Patient all treated alike (i.e. no cases)
the monster raving loony candidate (some Iranian sightings);
combines all the drawbacks of (A) and (B).
Agent / Patient treated alike as Transitive case Experiencer distinguished as Intransitive case
the orthodox Indo‐European approach; two cases, Nominative
(= Nonpatient) vs. Accusative (= Patient).
Agent / Experiencer treated alike as Nominative case Patient distinguished as Accusative case
looking glass logic – the rather widespread opposite
of (D); Agent vs. Nonagent.
Agent distinguished as Ergative case Experiencer / Patient treated alike as Absolutive case
a compromise solution – part (D), part (E). Also
Agent / (voluntary) Experiencer handled as Nominative case Patient / (involuntary) Experiencer handled as Absolutive case
What's more, many languages mix the above systems! For more detail, and further examples of exotic possibilities Zamenhof never considered, see… well, I used to cite an old favourite language typology textbook here, but I should probably be pointing people at the World Atlas of Language Structures.