> Can you give examples of what a "ergative–absolutive alignment" looks like, or case-stacking? It sounds fascinating...

It is fascinating! I can certainly try to explain these…

So, ergativity first. In English — as in pretty much all natural languages — the main verb of a sentence occurs alongside nouns, which describe the people or objects involved in the action. We call these nouns ‘arguments’ Some English verbs only take one argument, with the sentence becoming ungrammatical when given another:

I coughed. / *I coughed it.
He went. / *He went the hospital.

(The asterisk represents an ungrammatical sentence, by the way.)

By contrast, other verbs require two arguments, neither of which may be omitted:

He likes you. / *He likes. / *Likes you.
The verb requires nouns. / *The verb requires. / *Requires nouns.

We call the former set of verbs ‘intransitive’, and the latter ‘transitive’. (English also has numerous ‘ambitransitive’ verbs, which can be used with both one and two arguments; I’ll be ignoring those here.) Terminology varies, but I’ll call the argument before the verb the ‘subject’, and the argument after the verb the ‘object’.

Now, notice something interesting here. There are actually three different types of arguments here: the subject of an intransitive verb, the subject of a transitive verb, and the object of a transitive verb. But we’re calling the first two by the same name! We can do this because, in English, the intransitive subject aligns its behaviour with the transitive subject — in other words, English grammar treats them exactly the same way. Most prominently, both transitive and intransitive subjects occur in the same position in the sentence: before the verb. But there are other similarities. For instance, pronouns with two forms, such as ‘she’ vs ‘her’, have the same form across transitive and intransitive subjects, but use the other form for objects:

She coughs. / She likes you. / You like her.

Traditionally, the ‘she’ form is called ‘nominative’, and the ‘her’ form is called ‘accusative’, hence the name ‘nominative–accusative alignment’.

In the same sample sentences, note another similarity: verbs take the suffix ‘-s’ when the intransitive subject or the transitive subject is ‘he’, ‘she’, ‘it’ or a noun, but not when the object is any of these words. We call this ‘verbal agreement’: the verb is ‘agreeing’ with its subject. In English as with most European languages, verbs agree with the subject only. However, Basque is different: the verb has one agreement marker for its subject, but another one at the same time for its object. This is the ‘polypersonal agreement’ I mentioned earlier.

Now, we can start thinking about this system. In English, the intransitive subject aligns consistently with the transitive subject. But there’s no reason for this to be the case in all languages! We might imagine a language where the intransitive subject instead behaves just like the transitive object, rather than the transitive subject. If English were like this, the sample sentences above might look more like this:

Coughed me.
Went him.
He like you.
The verb require nouns.
Coughs her. / She like you. / You likes her.

Now, the sole intransitive argument appears after the verb, in the object form. Furthermore, the verb agrees with the object, not the subject. We call the grouping of intransitive argument+transitive object the ‘absolutive’, and the transitive subject by itself the ‘ergative’; thus, a system like this, where the intransitive argument behaves the same as the transitive object, is called ‘ergative–absolutive alignment’. This is the system found in Basque, as well as Georgian, Tibetan, Chukchi, Dyirbal and Salishan languages — but in no other European language.

Of course, things are a bit messier than the idealised picture I’ve given. For instance, most so-called ‘ergative’ languages are actually a mixture of alignments, with parts of the grammar using ergative alignment, but other parts using accusative alignment. I encourage reading more about this; the standard reference is R.M.W. Dixon’s monograph Ergativity, but there are others — it’s all very interesting!

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Next, case stacking. This one is thankfully easier to understand than ergativity. In many languages, nouns change their form to indicate their role in the sentence — pronoun pairs like ‘she’/‘her’ are an example of this in English. In Basque, as with most European languages, this is indicated through special ‘case-markers’, usually suffixes: ‘-a(k)’ for absolutive argument, ‘-ak/-ek’ for ergative argument, ‘-(e)(t)an’ for location, ‘-(a)(r)en’ for possessor, and so forth. In most European languages, each noun may have at most one case-marker. However, Basque allows some case-markers to be stacked: e.g. ‘gizona’ is ‘the man’, ‘gizonarena’ ‘of the man’, but then ‘gizonarenarena’ ‘of the one of the man’.

(Although, now that I look it up, there seems to be some controversy over whether Basque has ‘true’ case stacking; see http://www.incatena.org/viewtopic.php?p=1113792#p1113792 for discussion.)