ConLing - How does ergativity work?

Almost regardless of how you go about being interested in language, a basic understanding of morphosyntactic alignment is, in the words of Kanye, "something that you need to have." It's what takes you from thinking about syntax in terms of the languages you know, to the world of ever-ineffable true thematic relations, a realm where theta roles are made up, that's kept generations of high linguistics students up at night.

I apologize for making this.

If, for instance, you only know English, or even if you're a well-read pan-European polyglot comfortable with French, German, Latin, and Russian, the concept of a grammatical subject seems absolute and immutable, a truth of the universe up there with gravity. As soon as you work up the gall to crack open a grammar of Basque or Tagalog or Guarani, though, this sure footing starts slipping away beneath you, and you turn to the only place you can - morphosyntactic alignment. The best way to start thinking about this is with a nice, clean example  - ergativity.

In nominative languages, which include most of the languages you've heard of and indeed most languages in the world, there are two core theta roles: subject and object. To understand ergativity we first have to expand our repertoire to three - we'll call them Agent, Subject, and Object. Here, "subject" is not the subject you're likely used to - it refers to the one argument of an intransitive verb (that is, a verb with only one core argument - in English, just a subject). For a transitive verb, with two core arguments, the verber is the agent and the verbee is the patient.

Nope. Not that type of agent.

Ehh... not that type of patient either. We can only wish that morphosyntax had anything to do with Hugh Laurie.

Some examples might help:

John ate a bagel. 

You met John.

John slept.

I'm sure you'll be glad to know that this is a drastic oversimplification. I'm deliberately avoiding most types of clauses, and ignoring more subtle thematic relationships like Experiencers, Stimuli, Themes, and Instruments. For an understanding of ergativity, though, this model should work.

In a nominative language, subjects and agents are marked identically (with the nominative case), and differently from objects (accusative case). For instance, in English subjects and agents come before the verb and use nominative pronouns like he, and objects come after and use accusative pronouns like him.

He ate a bagel. 

You met him.

He slept.

In an ergative language, the subject flips allegiance.

If you take nothing else from this post, remember this chart.

If English was a purely ergative language, this is what we'd expect:

He ate a bagel. 

You met him.

Slept him.

Freaky.

So how does ergativity look in real languages? By far the most common way for it to appear is in morphology. In ergative languages that marks their nouns for case, like the infamous Basque, core arguments may take one of two cases - agents use the ergative case, while subjects and objects use the absolutive case.

Emakumeak du gizona ikusi.

"The woman (ergative) has seen the man (absolutive)."

Emakumea da etorri.

"The woman (absolutive) has arrived."

It's worth noting at this point that if one of the cases is zero-marked, it's likely to be the absolutive. In Basque, the ergative is marked with -k, and the absolutive with no suffix.

It's also possible for verbs to agree with subjects and objects in the same way. For instance, Yucatec Mayan has a set of verbal suffixes which indicate the subject of a transitive verb and the object of a transitive verb:

Hanen.

"I ate."

Ca yacunticen.

"You love me."

Cin yacunticech.

"I love you."

So yeah, that's morphological ergativity.

Here's a baby sloth yawning to reward you for your perseverance up to this point. We're more than halfway through.

The other place ergativity can show up is in syntax. It's worth noting that this is much less common, though. Syntactically ergative languages always show morphological ergativity, but the the opposite is far from true. Usually syntactic ergativity isn't a matter of basic word order within a clause, but of relationships from clause to clause. The most common example type for this has to do with coreference - the phenomenon of one noun having a part in two consecutive clauses. In English, if we say

Mother saw father and returned.

then who returned? Mother, of course! In a syntactically nominative language, agents are typically at the center of the discourse (again ignoring voice tricks like passives, which are not the default), so if you have one clause after another and an argument is left unstated in the second, it's assumed that the agent is carried over. Also, since agents and subjects occupy the same theta role, the agent of one clause can be the subject of the next without any fancy footwork to show the change in role. In Dyirbal, a syntactically ergative indigenous language of Australia, the opposite is true:

Nguma yabunggu buran banaganyu.

"Mother saw father and returned."

If you ask a Dyirbal speaker who returned, they'd say it was father! In Dyirbal, objects are more central to the discourse, so it's the object that's assumed to carry on, and since objects and subjects play the same theta role, the object of the first clause slots into the subject role in the next. This can also apply to relative clauses. In English if we say:

The man that sees.

the man is the agent of the seeing. The relative clause - "that sees" - contains just a bare verb, with no role marking. Because English is a nominative language, it's assumed that the man is the agent, and we'd have to use passive voice to make him the object. In a syntactically ergative language, though, it's again exactly the opposite - it would be assumed that the man was the object, and you might have to use trickery like an antipassive, or case marking on a relative pronoun, to make him the agent.

A last point about ergativity, from the standpoint of typology, is that it has a habit of showing up only partially - sometimes only in morphology, sometimes only in the third person, and sometimes only with some verbs (usually those involving low agency, like falling or being hungry). Many languages that were once described as ergative are now, with further investigation, being considered only halfway so.

So that's an overview of how ergativity basically works. It's by no means comprehensive, though - I strongly recommend taking a look at that U of Hawaii paper.

If your goal is to understand that world I mentioned at the top, where theta roles don't exist and thematic relationships are just a big soupy inconsistent mess, where do you go from here? Reading this was only the first step. Now go forth and explore active-stative languages, tripartite languages, fluid-S languages, trigger languages, other types of clauses and thematic roles and how they are marked, and whatever else your big grammarian brain can find!

1 May: Correction, courtesy of Reddit's /u/folran:

"A ≠ agent, and P ≠ patient! A and P are syntactic roles, not semantic ones. Their definition can be based on semantic roles, but they are not semantic roles themselves."

This one stung because I knew it, but I got sloppy and forgot to make the distinction when I wrote this. Throughout this post I keep referring to the syntactic roles as Agent, Patient, and Subject, but they should properly be A, P, and S. Agent, patient, and subject are semantic roles that remain even in the face of syntactic switcheroos, and A, P, S are the syntactic slots usually occupied by those roles. For example, in

A bagel was eaten by John.

the bagel occupies the S syntactic role, even though it's still semantically a patient. Case marking is based on syntactic roles.

Further reading:
Wikipedia
U of Hawaii
Syntactic ergativity 
Morphosyntactic alignment
Thematic relation
Theta role
Grammar of Basque
Yucatec Mayan verb suffixes