Every Esperanto grammar book I've seen has insisted that "unu" is never to be given an accusative ending. This makes sense at first glance, since none of the other numbers decline.

But it seems to be well-established by EO grammarians (no less than Bertilow) the "unu" in its plural form, as a pronoun, "unuj", can indeed take on the final -n. Tekstaro has 12 hits for "unujn". However, "unun" as a direct-object pronoun (not the cardinal number) remains all but unacceptable for some reason (3 hits on Tekstaro from 2 sources).

Examples:

unuj objektoj venis en unujn manojn, aliaj en aliajn manojn - Fabeloj de Andersen

kaj unujn savu, eltirante ilin el la fajro; kaj aliajn kompatu kun timo - Nova Testamento

So this appears to place "unu" on a rather exclusive list of "defective" Esperanto words. Can anyone suggest why?

Very sharp observation.

Let's do a little Esperanto 101 (as the Americans would say).

Esperanto words may be divided into two groups.

Group 1. Compound words that are composed of two or more elements of which the last one must be a grammatical ending. Most Esperanto words find themselves in this group.

In this group you may find words based on the same root that may have almost any grammatical ending; rapida, rapido, rapide, rapidi - all have real world application. But in the nature of the world there is little call for the adverb table, or the verb tabli compared to the noun tablo.

Group 2. Single element words (nude roots) - like la, pri, kiel, unu, dum, ĉiu. These words can assume different grammatical roles, there being no explicit role marker. They can to varying degrees also be combined with a grammatical endings. Some are never so combined, some rarely and some frequently, and when so combined express something different to the nude root.

Unu is not defective. It is a Group 2 word (solstariva radiko) that can sometimes be used with a grammatical ending, but then only some endings.

So in the wider framework of Esperanto word formation 'unu can be considered as quite ordinary'.

I have never heard of 'unuj' ever. That said, there is a great deal in Esperanto that lies outwith my experience.

I'd have thought that 'unu' would take the form 'unuo' which easily becomes 'unuoj' in the plural.

I'd be grateful for any links or references for 'unuj'.

Influence by the cardinal number, see PMEG 23.1.3. Specialaj uzoj de unu

Kelkaj volas, ke unu povu ricevi N-finaĵon, kiam ĝi montras individuecon. Kelkaj eĉ eksperimente praktikas tion: *Mi trovis nek unun, nek la alian.* Estas vero, ke la regulo, kiu malpermesas *unun*, sed permesas unujn, estas stranga el vidpunkto de logiko. Sed ĝi estas tre bona el vidpunkto de praktika uzado. En la praktiko la nombra, unikeca kaj individueca uzoj de unu ne estas rigore distingeblaj. Estas multaj limokazoj, kaj tial la simpla regulo, ke unu neniam havu N-finaĵon, estas tre praktika. Oni ne bezonas cerbumi ĉiufoje, ĉu unu montras nombron, unikecon aŭ individuecon. Unuj tamen ĉiam montras individuecon aŭ unikecon, kaj tial ne estas problemo uzi post ĝi N-finaĵon, se la frazrolo tion postulas. Uzado de *unun* kaŭzus nur konfuzon sen alporti multe da praktika utilo.

malglatamelo:I have never heard of 'unuj' ever. That said, there is a great deal in Esperanto that lies outwith my experience.

[...]

I'd be grateful for any links or references for 'unuj'.

Tekstaro is full of them. A few examples:

kun tremo ili diris unuj al la aliaj: Kial Dio faris tion al ni? -Nova Testamento

Granda popolamaso nun kriis tie, unuj puŝis la aliajn. -Fratoj Grimm, Elektitaj Fabeloj.

sudanglo:
Unu is not defective. It is a Group 2 word (solstariva radiko) that can sometimes be used with a grammatical ending, but then only some endings.

So in the wider framework of Esperanto word formation 'unu can be considered as quite ordinary'.

Thanks, sudanglo. I'm fairly familiar with the distinction between the two word groups. However, even now, "unu" seems to be atypical. Other Group 2 words, once they are combined with grammatical endings -o, -a, can be further given both accusative and/or plural endings. Dumaj, nunan, pliajn, and so on.

Problem: Unu does not take on -o or -a before being declined, unlike the other words (*plijn is malformed). Instead, the plural markers are just added directly. "Unuj" does not mean "unuaj". This characteristic would make "unu" more comparable with the relative pronouns "ĉiu", "kiu", etc., which also are declined directly.

Kelkaj volas, ke unu povu ricevi N-finaĵon, kiam ĝi montras individuecon. Kelkaj eĉ eksperimente praktikas tion: *Mi trovis nek unun, nek la alian.* Estas vero, ke la regulo, kiu malpermesas *unun*, sed permesas unujn, estas stranga el vidpunkto de logiko. Sed ĝi estas tre bona el vidpunkto de praktika uzado. En la praktiko la nombra, unikeca kaj individueca uzoj de unu ne estas rigore distingeblaj. Estas multaj limokazoj, kaj tial la simpla regulo, ke unu neniam havu N-finaĵon, estas tre praktika. Oni ne bezonas cerbumi ĉiufoje, ĉu unu montras nombron, unikecon aŭ individuecon. Unuj tamen ĉiam montras individuecon aŭ unikecon, kaj tial ne estas problemo uzi post ĝi N-finaĵon, se la frazrolo tion postulas. Uzado de *unun* kaŭzus nur konfuzon sen alporti multe da praktika utilo.

I didn't see that bit of the PMEG, thanks. I suppose there is something to be said for the response, "It's too complicated and it's not helpful." Admittedly, the number of realistic contexts where confusion could result from this rule, is practically nil.

Perhaps I'm being pedantic, but I think there's still a "gap" - in principle, if one generalises from other accusative use, and is able to perfectly distinguish between the cardinal and the pronoun, one would be justified in using the pronoun, "unun".

Tempodivalse:Problem: Unu does not take on -o or -a before being declined, unlike the other words (*plijn is malformed). Instead, the plural markers are just added directly. "Unuj" does not mean "unuaj". This characteristic would make "unu" more comparable with the relative pronouns "ĉiu", "kiu", etc., which also are declined directly.

Exactly. "unu" cannot be compared to e.g. "nun", because it takes inflectional morphemes (j, n) without taking a derivational morpheme (o, a, e).

Apparently nominally inflected words in Esperanto come in various flavours:

[list=1]
Bare singularia tantum: These are for instance the personal pronouns. They add the accusative ending directly to the stem and don't have a plural.
Bare: These are for instance pronouns like kiu, kia, etc: They have both plural and accusative and add the endings directly to the stem.
Defective bare: The word unu. It adds the inflections directly to the root and lacks an overt accusative singular.
Derivational-inflectional: These are the "standard" words. They need to take first a derivational ending and then they may take an inflectional one:

• Fully inflectional: Whenever the derivational ending is -o or -a.
  
• Singularia tantum: Whenever the derivational ending is -e.

[/list]

One could get by very well never using "unuj". It's not at all common, and I wouldn't have a problem with "ili diris unu al la aliaj"...(understanding that "unu" refers separately to each individual of "ili", who then speaks to the others)

PAG, interestingly, disagrees with PMEG:

Plena Analiza Gramatiko:Sed tia konfuzo inter du malsamaj funkcioj ne pravigas la strangan formon de objekta nominativo; kaj nur la tradicio povis tiun stranĝaĵon, ke unu havas, en la sama funkcio, akuzativon pluralan, sed ne havas akuzativon singularan. (4-a eldono, pĝ. 88)

Additionally:

Plena Analiza Gramatiko:...kaj tiu nevarieco povas okazigi malklaraĵon: "unu el viaj fratoj lasu ĉe mi!"

I have to agree with PAG. Difficulty in distinguishing between the pronoun and the cardinal numeral is a poor justification for avoiding the accusative. Defectivity of certain parts of speech, in a language famous for not having exceptions, should not be encouraged.

Tempodivalse:PAG, interestingly, disagrees with PMEG:

Plena Analiza Gramatiko:Sed tia konfuzo inter du malsamaj funkcioj ne pravigas la strangan formon de objekta nominativo; kaj nur la tradicio povis tiun stranĝaĵon, ke unu havas, en la sama funkcio, akuzativon pluralan, sed ne havas akuzativon singularan. (4-a eldono, pĝ. 88)

Additionally:

Plena Analiza Gramatiko:...kaj tiu nevarieco povas okazigi malklaraĵon: "unu el viaj fratoj lasu ĉe mi!"

I have to agree with PAG. Difficulty in distinguishing between the pronoun and the cardinal numeral is a poor justification for avoiding the accusative. Defectivity of certain parts of speech, in a language famous for not having exceptions, should not be encouraged.

Case marking seems to have several exceptions in Eo. For instance adverbs mark accusative of direction (hejme -> hejmen), but they don't mark accusative when they are a direct object:

Zamenhof:Vi vidis multe, sed vi ne konservis