[gordonguthrie]

And the case system differentiates between indirect objects of motion and position (like Polish) and doesn't have a genitive (which German does)

And also English has like 12-16 vowel sounds and Esperanto (like the Slavics) has half that many

[AprilArcus]

[Not trying to argue against Esperanto's Slavic influence, just having fun and being pedantic]

Yes, the fact that English has a tense/lax vowel distinction rather than long/short is very unusual.

My English dialect (Providence) has five tense-lax pairs (ɑː/æ, eɪ/ɛ, iː/ɪ, oʊ/ɔ, uː/ʊ), a central unrounded vowel (ʌ), and three diphthongs (ɔɪ, aɪ, aʊ), so arguably fourteen "vowel sounds" (General American merges ɑː and ɔ, bringing the total down to thirteen). Which is a lot, by any measure! And many European languages do have much simpler systems, famously Spanish.

However, Esperanto has its five pure vowels as well as six dipthongs (ai, ei, oi, ui, ou, eu), for a total of eleven, which is hardly "half as many"!

[squiggleblaz]

Concerning the number of vowels a language has: https://wals.info/chapter/2 does indeed class 7-14 as a lot. But it's very hard to count vowels in any kind of neutral way.

Concerning Esperanto, it only has two diphthongs - aw and ew. Aj, ej, oj and uj are actually just sequences of a vowel and consonant. For an comparison, consider is English "ye-" as in "yes" is a consonant and a vowel, but the same sound (ie-) in Spanish or Finnish is a diphthong.

What constitutes a diphthong vs a sequence is specific to an analysis of a given language. In Esperanto, we can see that "diversa sono" and "diversaj sonoj" differ just by the addition of a -j sound and we always teach that it's an affix. It isn't the deletion of the /a, o/ and its replacement by a diphthong /ai, oi/ which would destroy the simplicity of Esperanto and confuse anyone whose native language wasn't English.

Additionally, the only hole is -ij, which is missing for good phonotactic reasons - the sounds aren't distinct enough - which is extremely common and shared by a missing ji- so it can't fairly tempt us to analyse it as a diphthong.

This is in contrast to a language like English. /-j/ cannot be added after just any vowel. Moreover, whenever it occurs, it's part of the same root/morpheme as the previous vowel sound. So there's no advantage to analysing them separately (altho some people try to cut back on the number of English vowels, perhaps to only six, by counting the ones you note as ah/a, ey/e, iy/i, ow/o, uw/u, ʌ/(ʌr), oy, ay, aw - but this is normally rejected as being too weird).

English speakers often seem to teach Eo /aj, oj, uj, ej/ as diphthongs simply because of the phonetic properties of English.