Phonotactics
A syllable in Esperanto is generally of the form (s/ŝ)(C)(C)V(C)(C). That is, it may have an onset, of up to three consonants; must have a nucleus of a single vowel or diphthong (except in onomatopoeic words such as zzz!), and may have a coda of zero to one (occasionally two) consonants.
Any consonant may occur initially, with the exception of j before i (though there is now one word that violates this restriction, jida "Yiddish" which contrasts with ida "of an offspring").
Any consonant except h may close a syllable, though coda ĝ and ĵ are rare in monomorphemes (they contrast in aĝ’ 'age' vs. aĵ’ 'thing'). Within a morpheme, there may be a maximum of four sequential consonants, as for example in instruas "teaches", dekstren "to the right". Long clusters generally include a sibilant such as s or one of the liquids l or r.
Geminate consonants generally only occur in polymorphemic words, such as mal-longa "short", ek-kuŝi "to flop down", mis-skribi "to mis-write"; in ethnonyms such as finno "a Finn", gallo "a Gaul" (now more commonly gaŭlo); in proper names such as Ŝillero "Schiller", Buddo "Buddha" (now more commonly Budho); and in a handful of unstable borrowings such as matĉo "a sports match". In compounds of lexical words, Zamenhof separated identical consonants with an epenthetic vowel, as in vivovespero "the evening of life", never *vivvespero.
Word-final consonants occur, though final voiced obstruents are generally rejected. For example, Latin ad "to" became Esperanto al, and Polish od "of, by, than" morphed into Esperanto ol "than". Sonorants and voiceless obstruents, on the other hand, are found in many of the numerals: cent "hundred", ok "eight", sep "seven", ses "six", kvin "five", kvar "four"; also dum "during", eĉ "even". Even the poetic elision of final -o is rarely seen if it would leave a final voiced obstruent. A very few words with final voiced obstruents do occur, such as sed "but" and apud "next to", but in such cases there is no minimal-pair contrast with a voiceless counterpart (that is, there is no *set or *aput to cause confusion with sed or apud). This is because many people, including the Slavs and Germans, do not contrast voicing in final obstruents. For similar reasons, sequences of obstruents with mixed voicing are not found in Zamenhofian compounds, apart from numerals and grammatical forms, thus longatempe 'for a long time', not *longtempe. (Note that /v/ is an exception to this rule, as it is in the Slavic languages. It is effectively ambiguous between fricative and approximant. The other exception is /kz/, which is commonly treated as /ɡz/.)
Syllabic consonants occur only as interjections and onomatopoeia: fr!, sss!, ŝŝ!, hm!.
All triconsonantal onsets begin with a sibilant, s or ŝ. Disregarding proper names, such as Vladimiro, the following initial consonant clusters occur:
- Stop + liquid – bl, br; pl, pr; dr; tr; gl, gr; kl, kr
- Voiceless fricative + liquid – fl, fr; sl; ŝl, ŝr
- Voiceless sibilant + voiceless stop (+ liquid) – sc, sp, spl, spr; st, str; sk, skl, skr; ŝp, ŝpr; ŝt, ŝtr
- Obstruent + nasal – gn, kn, sm, sn, ŝm, ŝn
- Obstruent + /v/ – gv, kv, sv, ŝv
And more marginally,
- Consonant + /j/ – (tj), ĉj, fj, vj, nj
The affectionate suffixes -ĉj- and -nj-, which retain remnants of the Slavic palatalized consonants, may very occasionally be used as words in their own right, as in mia ĉjanja popolo "my dear nation", in which case they may be word initial and not just syllable initial.
Although it does not occur initially, the sequence dz is pronounced as a an affricate, as in edzo "a husband" with an open first syllable, not as *.
In addition, initial pf- occurs in German-derived pfenigo "penny", kŝ- in Sanskrit kŝatrio "kshatriya", and several additional uncommon initial clusters occur in technical words of Greek origin, such as mn-, pn-, ks-, ps-, sf-, ft-, kt-, pt-, bd-, such as sfinktero "a sphincter" (which also has the coda nk). Quite a few more clusters turn up in sufficiently obscure words, such as tl in tlaspo "Thlaspi" (a genus of herb), and Aztec deities such as Tlaloko "Tlaloc". (The /l/ phonemes are presumably devoiced in these words.)
As this might suggest, greater phonotactic diversity and complexity is tolerated in learnèd than in quotidian words, almost as if "difficult" phonotactics were an iconic indication of "difficult" vocabulary. Diconsonantal codas, for example, generally only occur in technical terms, proper names, and in geographical and ethnic terms: konjunkcio "a conjunction", arkta "Arctic", istmo "isthmus".
However, there is a strong tendency for more basic terms to avoid such clusters, although cent "hundred", post "after", sankta "holy", and the prefix eks- "ex-" (which can be used as an interjection: Eks la reĝo! "Down with the king!") are exceptions. Even when coda clusters occur in the source languages, they are often eliminated in Esperanto. For instance, many European languages have words relating to "body" with a root of korps-. This root gave rise to two words in Esperanto, neither of which keep the full cluster: korpuso "a military corps" (retaining the original Latin u), and korpo "a biological body" (losing the s).
Many ordinary roots end in two or three consonants, such as cikl-o "a (bi)cycle", ŝultr-o "a shoulder", pingl-o "a needle", tranĉ-i "to cut". However, these roots do not normally entail coda clusters except when followed by another consonant in compounds, or with poetic elision of the final -o. Even then, only sequences with decreasing sonority are possible, so while poetic tranĉ’ occurs, *cikl’, *ŝultr’, and *pingl’ do not. (Note that the humorous jargon Esperant’ does not follow this restriction, as it elides the grammatical suffix of all nouns no matter how awkward the result.)
Within compounds, an epenthetic vowel is added to break up what would otherwise be unacceptable clusters of consonants. This vowel is most commonly the nominal affix -o, regardless of number or case, as in kant-o-birdo "a songbird" (the root kant- "to sing" is inherently a verb), but other part-of-speech endings may be used when -o- is judged to be grammatically inappropriate, as in mult-e-kosta "expensive". There is a great deal of personal variation as to when an epenthetic vowel is used.
Read more about this topic: Esperanto Phonology