Esperanto is quite regular, right? No "am, is, are" or "sum, es, est, sumus, estis, sunt". It doesn't make sense for an auxlang to be irregular, after all.

But ...

Like many (OK, several) Esperantists I thought I'd take a look at Volapük, and I was a bit surprised to see that it's regular in ways Esperanto isn't.

First, pronouns. In Esperanto, the plural pronouns are not derived from the singular. Mi, ni; vi; li-ŝi-ĝi, ili.

In Volapük, the plurals are formed by adding -s, just as with nouns or adjectives. Ob, obs; ol, ols; om-of-on, oms-ofs-ons.

Then there are hieraŭ, hodiaŭ, morgaŭ. In Volapük these are ädelo, adelo, odelo. "Del" is the word for "day", the -o is the adverbial ending, and the ä-, a-, and o- prefixes indicate past, present and future - the same prefixes used on verbs. Quite regular.

So ...

Is this a feature or a bug for Esperanto, which after all displaced Volapük historically? Do a few irregularities make it more like a natlang?

It's not a bug, and I wouldn't even call it an irregularity. At most, it's declining to create words in a systematic way, in favor of a less systematic way.

You could argue that the pronouns would have been better off differently. But as far as words like hodiaŭ, morgaŭ, etc, Esperanto words were chosen to resemble words that speakers of European languages were familiar with, making it easier to memorize.

One of the reasons why Volapuk never succeeded on a wide scale is that Volapuk words bear no resemblance whatsoever to any words anyone has heard of (even though they are derived from European Roots). It was regular but very complicated and the words were hard to memorize. (Because it was totally obvious to you on reading the name "volapuk" that it means "World speak", right?)

Esperanto makes more of a compromise between naturalism and contrived rules and systems. Every constructed language finds itself somewhere on that continuum; I think Esperanto has done well because it struck a good balance between naturalism and systematic rules. Interlingua is a good example of a more naturalistic language. Volapuk is less naturalistic, Loglan/Lojban is even less naturalistic still.

On the use of regular systems to construct words, playing the devil's advocate, too much regularity can also add difficulty. All Esperanto letters are named by taking the consonant sound and adding -o, or just pronouncing the vowels. That means that it's hard to spell something over the phone because bo and do might sound very similar, or po and to. Some people have advocated a modified alphabet pronunciation for situations like that. There have been some complaints about dekstra/maldekstra as well; in situations where you're giving directions and you can't hear well, the 'mal' could easily be lost in the shuffle. Some people have suggested using "liva" instead of "maldekstra" in those situations. I don't use it myself, I just pronounce the MAL part very loudly and emphatically if I'm worried about being understood (accompanied by hand motions if it's in person)

Similarly, the difference between of and ofs might not be easy to hear if sound quality is bad.

Forigite

Hyperboreus:
But I don't see there an issue, it is fine as it is now.

Just to clear, my post above was not intended as a reform proposal!

erinja:It's not a bug, and I wouldn't even call it an irregularity. At most, it's declining to create words in a systematic way, in favor of a less systematic way.

Hmmmm. So where do you stand on biologio vs vivscienco and similar?

It was regular but very complicated and the words were hard to memorize.

I've found it fairly easy when I can deduce the etymology. Some of Volapük is apriori, though. The numbers were a bear at first.

(Because it was totally obvious to you on reading the name "volapuk" that it means "World speak", right?)

Of course!

On the use of regular systems to construct words, playing the devil's advocate, too much regularity can also add difficulty.

I've found Volapük's agglutination is more difficult than regularity. A five-syllable word probably has two prefixes and two suffixes. Very dense. I have to slow down and parse it out. But then some natlangs are like that, aren't they? Basque?

Bruso:

erinja:It's not a bug, and I wouldn't even call it an irregularity. At most, it's declining to create words in a systematic way, in favor of a less systematic way.

Hmmmm. So where do you stand on biologio vs vivscienco and similar?

I am generally not in favor of creating neologisms when existing roots will do.

...however, I make an exception when it comes to technical terms. Biology and Life Science are not exactly the same thing; Life Science is a broader category, encompassing more sciences than just biology.

I say "biologio".

If Esperanto would ever to be used on a large scale, you need to have technical terms. I know this is a favorite topic of Sudanglo, one of our regular forum readers. If you try too desperately to avoid neologisms, it might not be helpful to anyone. Sometimes you need to distinguish between similar things, and at a certain point adding more suffixes and adjectives makes it more confusing than it needs to be.

I also see room to have a 'technical term' that not everyone might now, and a 'lay term' that may be less precise and in some ways less correct, existing side by side. We have this in national languages, and I see no reason why it wouldn't work for Esperanto as well.

Compare "myocardial infarcton" with "heart attack", for example.

erinja:
Similarly, the difference between of and ofs might not be easy to hear if sound quality is bad.

When I was learning Russian, i had various occasions to be listening to....radio broadcasts for which I got a transcription later. Anyway, you would not believe how often the words for 5 and 6 sound alike (they're pronounced roughly "pyaht" and "shehst" ). Another broadcast had the number 250 ("DVYEH-stee-Peed-dee-SYAHT" ) and it sounded exactly like the English sentence "This is a fish", hand to God! Both were a bit staticky, so, yeah, I can attest to what Erinja said above!

Hyperboreus:I think some asymmetries add flavour, but I was also a bit astonished to find the pronouns less symmetrical than the rest.

Interrogative, relative and demonstrative pronouns only distinguish animate/inanimate: kiu/kio, tiu/kio
Personal pronouns distinguish sexus, but only in 3rd person and only in singular: li ŝi ĝi
2nd person personal pronouns do not distinguish numerus: vi
Plural forms have nothing to do with singular forms: mi -> ni, vi -> vi, li ŝi ĝi -> ili

Hypothetically this could have been arranged more symmetrically, e.g.:
mu
vu
ĝu, ĝo
muj
vuj
ĝuj, ĝoj

-u for animate, -o for inanimate like with the rest of the pronouns. No "mo, vo, moj, voj" as inanimate objects rarely speak or are spoken to (*). If you want to emphazise "female" use the -in- as always: minu, vinu, ĝinu, etc...

But I don't see there an issue, it is fine as it is now.

--
(*) Unless I speak e.g. to my compiler: "Ĉu vo neniam komprenas kion mu volas." Or when HAL spoke: "Mo bedaŭras, Dave, mo ne povos fari tion."

Except that the problem with ĝo (and in the rare occasions mo and vo) is that they are the names of the letters ĝ, m, and v. Granted the context would make it clear you weren't denominating the letters, but I can imagine Zamenhof deciding that homophonic "roots" like that are best avoided.

Bruso:I've found Volapük's agglutination is more difficult than regularity. A five-syllable word probably has two prefixes and two suffixes. Very dense. I have to slow down and parse it out. But then some natlangs are like that, aren't they? Basque?

Georgian (post-Soviet, not American South ) is like that in its (IMO nightmarish) verb system. The various verb forms can have up to 7 morphems (some syllabic, others one letter).

Interesting story about Georgian (and Turkish, come to think of it) and Esperanto poo-pooers:
I was told once by a linguistics professor (PhD in Linguistics; how he managed that without knowing the following fact is beyond me) that Esperanto couldn't be a "real" language because "real" languages don't put the plural marker before the case marker. I had to laugh in his face and dared him to tell a Georgian or a Turk their languages weren't "real"....yep, they do exactly what Esperanto does: plural marker before the case markers.

Personal pronouns are the most frequently used words in any language and they are few, so it is very easy to learn them. Claim for the regularity here is not very valid, same for hieraŭ, hodiaŭ, morgaŭ - only three words to remember. If there were a hundred of them or more, then regularity would be welcome. After all Esperanto is not a code, but a living language, and life is irregular anyway. It is tenfold more regular than any native language I can think of.