(This article uses a whole lot of original research and Ardalambion notes. It would be tempting to post this as a formal article. But without additional peer review, it seems safer for now to keep this as a user space page.)
The issue of Quenya phonotactics is important to understand when dealing with Quenya grammar or when adapting new loanwords to Quenya, as Quenya has very strict phonotactics and only certain combinations of sounds are allowed in any given word.
In this article, for the sake of simplicity, the common Quenya spelling k will be avoided, and c will be used instead. Additionally, the spellings cw kw will not be used (only qu), and the spellings cs ks will not be used (only x).
 Syllable structure
Generally speaking, a Quenya word can only begin with no more than one consonant and end with no more than one consonant. Consonant clusters are allowed inside a word, but they can be no longer than two consonants, and only a limited selection of clusters are permitted.
As for vowels, a word may begin or end with a vowel, and vowels may border each other in syllable hiatus. However, once again, certain combinations are prohibited.
 Initial consonants
If a Quenya word begins with a consonant, that consonant may only be one of the following.
- c f h hl hr hw hy l m n ñ ñw ny p qu r s t ty v w y þ
- The consonants hl hr hw hy ñw ny qu may look like clusters, but (at least word-initially) they are homogeneous single consonants.
- hl hr hw hy are voiceless versions of those sounds (where the h represents devoicing)
- ñw is a labialized ñ
- ny is a palatalized n/ñ.
- qu is a labialized c.
- ty is a palatalized c/t.
- The consonants ñ ñw are lost in Middle-earth after the Second Age, having been replaced by n nw. The initial consonant nw is only possible after the Second Age and cannot occur initially before then. It should therefore be understood that word-initial ñw and nw are mutually exclusive within any given Quenya dialect.
- The consonant w is lost word-initially in Middle-earth after the First Age, having been replaced by v.
- The consonant þ is lost among the Noldor even before the Rebellion of the Noldor. It is retained by Vanyar, and by Fëanor and his family into the First Age, but is gradually abandoned even by them after Fëanor dies.
- A single word, cyermë, is attested with initial cy, but it is used only in Númenor (where the use of Quenya is largely ceremonial), and is suspected to be a corruption of a different Quenya word that does not begin with cy. cy is not believed to exist in any native Quenya dialect, as it would assimilate to ty.
 Final consonants
The list of allowed word-final consonants is even shorter.
- l n r s t
- Many Quenya words inflect the dative dual case to end with nt, and this cluster in this situation is the only known cluster allowed at the end of a Quenya word, and it seems not to be otherwise allowed.
 Intervocalic consonants
Here's the biggest part of the list. Many combinations are attested, and still more combinations are theoretically possible.
 Single consonants
All initial consonants, plus:
- *fy ly my *py ry sy *vy z zy þ *þy
- The forms cy ñy are prohibited, properly replaced with ty ny respectively.
- The consonants hl hr ñ ñw are primarily just word-initial consonants, though they can occasionally be seen in compound words where the non-initial morpheme begins with one of these consonants. However, many old compound words do not have these consonants word-internally because of the different rules governing the developing of initial and intervocalic consonants. In particular:
- hl usually came from an older sl, which instead became ll between two vowels.
- Likewise, hr usually came from an older sr, which instead became rr between two vowels.
- Initial ñ evolved from both ñ and ng, but ng seems to have been preferred for both intervocalically.
- Initial ñw evolved from both ñw and ngw, but ngw seems to have been preferred for both intervocalically.
- The consonant z is lost among the Noldor before the Rebellion of the Noldor, but is retained by the Vanyar.
- Intervocalic þ was replaced with s the same way word-initial þ was replaced.
- The consonants fy py vy seem theoretically possible, but are unattested. The only reason they are shown is because my is attested.
- zy and þy are theoretically possible, but would be exclusive to Vanyarin even before the Noldor ever returned to Middle-earth. They largely don't concern more common Quenya study.
- The existence of intervocalic d is attested only in Vanyarin Aldudénië. It is unknown whether this is a quirky dialectal feature of Vanyarin or an irregular Vanyarin word, but otherwise d is known never to exist by itself intervocalically in common Quenya words. In Quenya historic development, intervocalic d became ð and then z, and then among the Noldor z became r.
 Consonant clusters
All attested and plausibly imagined:
- cc *ff *hh ht hty lc lb *lby ld *ldy lk ll lm *lmy lp *lpy lqu lt *lty lv lw mb *mby mm mn *mny mp *mpy nc nd ndy ng ngw nn nqu nt nty pp ps pt *pty rc rd *rdy rm *rmy rn *rny rp *rpy rqu rr rs *rsy rt *rty rv *rvy rw rþ *rþy ss sc sp *spy squ st sty sw ts tt tw v w x zm *zmy *þw þþ
- It does not appear possible to double a palatalized consonant (e.g. there is no nny, etc.). This raises the question of whether palatalized consonants can be interchangeably single or double by their very nature.
- In addition to the already-prohibited cy ñy, the form ngy also cannot occur, and is replaced with ndy. However:
- ndy is kept only by the Vanyar, and the Noldor and Middle-earth replace it with ny.
- lb seems to be exclusive to the Vanyar. Noldor and Middle-earth typically use lv instead.
- Though hw qu can exist as single labiovelar consonants, other combinations with w can only be clusters with a labiovelar approximant.
- It is uncertain whether the clusters fw mw pw vw are permitted at all. It seems unlikely, considering that the consonants f m p v are already labial, and w is labiovelar, which in its proximity would essentially make the clusters velarized labials. Considering that Quenya has no other mechanism of velarization (and the sarati-era phoneme ȝ completely vanished), and also considering that there is already a separate labiovelar series, it seems likely that these clusters would be prohibited and substituted with something else.
- It should be understood that x is always a consonant cluster.
A vowel forms the nucleus of each syllable. There are three classes of vowels: short, long and diphthong.
Not complicated. Any syllable can have one of these as its nucleus.
- a e i o u
Slightly more complicated. Long vowels are not permitted before consonant clusters, and are not normally allowed before a single consonant at the end of a word. There are some very irregular exceptions, such as the words Kalórmë and Palantír.
- á é í ó ú
A tad more complicated. Diphthongs generally obey the same rules as long vowels, i.e. they can't exist before a consonant cluster. However, many grammatical inflections allow diphthongs to exist before a final consonant.
- ai au eu iu oi ui
- The diphthongs ei ou are prohibited. ei becomes í or i, depending on the circumstances. In an irregular historical development, the ai in Vairë's name evolved from an ei. ou is too poorly attested to tell, but it probably becomes either ú or au.
Knowing the limits of what sounds are permitted in a Quenya word, one must then know how combinations of sounds are usually assimilated in the language.
 Initial consonants
When adapting a loanword to Quenya, the general pattern for initial consonants is:
- b becomes v.
- bl becomes l.
- br becomes r.
- cl becomes cal or possibly hl.
- cr becomes car or possibly hr.
- d becomes l.
- dr becomes r.
- dy becomes y.
- fl becomes hl or possibly fal.
- fr becomes hr or possibly far.
- g becomes silent.
- ly becomes y.
- pl becomes pal or possibly hl.
- pr becomes par or possibly hr.
- ry becomes y.
- sc becomes h.
- scr becomes hr.
- sl becomes hl.
- sp becomes f.
- spl: see fl.
- spr: see fr.
- squ becomes hw.
- sr becomes hr.
- st becomes þ (Noldorin s).
- str: see þr.
- sw becomes hw.
- sy becomes hy.
- tl becomes tal or possibly hl.
- tr becomes tar or possibly hr.
- z becomes s.
- þr (thr) becomes hr.
- Non-Quenya ch (as in church) should be adapted as ty.
- Non-Quenya ch (as in loch) should be adapted as h.
- Non-Quenya j (as in John) should be adapted as y.
- Non-Quenya lh should be adapted as hl.
- Non-Quenya rh should be adapted as hr.
- Non-Quenya sh (as in ship) should be adapted as hy.
- Non-Quenya wh should be adapted as hw.
- Non-Quenya wr could potentially be adapted as ur, or r if the w is unambiguously silent.
 Intervocalic single consonants
Many single consonants fare no better between vowels.
- b becomes v. If the b absolutely must remain a plosive, p can be used instead.
- Whether intervocalic d is stable among the Vanyar is uncertain. But in most regular cases even among the Vanyar, d becomes ð temporarily, but this does not last. See ð for more details. If the d absolutely must remain a regularly-manifested plosive, t can be used instead.
- ð (voiced th) becomes z. See z for more details.
- g becomes ȝ temporarily, but this does not last. See ȝ for details. If the g absolutely must remain a plosive, c can be used instead.
- ȝ becomes silent. This brings the two neighboring vowels into hiatus with each other, and not all vowel hiatus combinations are stable—see that section of this article for details. If the ȝ absolutely must not be silent, then h can be used instead.
- s is a mixed situation. In historic Quenya development it becomes z—see z for details. But after the Noldor merge þ with s, s becomes stable between vowels. A Noldorin-style adaptation permits this, but likely not a Vanyarin-style one.
- w is a more complicated case, as depending on the circumstances, it can become v, silent, or not change from w.
- aiw does not change.
- áw becomes au.
- awa becomes öa.
- éw becomes eu.
- ewa becomes öa.
- íw becomes iu.
- ow does not change.
- uw becomes u.
- wo becomes o.
- wu becomes u.
- In most other cases, w becomes v.
- y is also somewhat complicated, but not as complicated as w.
- áy becomes ai.
- aya becomes ëa.
- ey seems to become y. (What does this mean for éy?)
- iy becomes i.
- óy becomes oi.
- oya becomes ëa.
- úy becomes ui.
- yi becomes i.
- In most other cases, y does not change.
- z is kept among the Vanyar, but it becomes r among the Noldor. In Middle-earth it's slightly trickier—for z that absolutely must remain a fricative, s can be used instead.
- þ (voiceless th) remains among the Vanyar, but it becomes s among the Noldor and in Middle-earth.
 Consonant clusters
Many, many potential clusters are prohibited. The simplest way of assimilating many of them is just to insert the vowel e between them, especially with noun case endings or other situations where clarity is especially important. However, there are some accepted frequent assimilations that commonly arise.
- cn becomes nc.
- ln becomes ld.
- pn becomes mp.
- qu before a consonant becomes cu.
- rl becomes ll.
- tn becomes nt.
 Historically attested
These are other assimilations attested from the time just before sarati that can be useful for adapting loanwords or compound words, but it is not entirely certain whether they are still productive in inflections—inflect at your own risk. This list also includes assimilations involving the extinct phonemes ð ȝ, which are useful only as cues for adaptation. If adapting loanwords with b d g and those consonants come after the beginning of a word or after a vowel, substitute v ð ȝ respectively; ð ȝ further assimilate the same way they do as single intervocalic consonants, with ȝ becoming silent after all other considerations have been met.
- cm becomes ngw.
- ct becomes ht.
- ðl becomes ll.
- ðm becomes nw. Note this would have happened historically before ð becomes z even among the Vanyar.
- ðr becomes rr.
- Except where otherwise indicated, ȝ disappears, except that if it occurs before a consonant and after a short vowel, where the short vowel is lengthened.
- ȝm becomes ngw.
- l after a consonant usually switches places before it.
- Though ln becomes ld in inflection, it can also be adapted as ll.
- lr becomes ll.
- ls becomes ll.
- nl becomes ll.
- nm becomes nw or mm.
- nr becomes rr.
- ns becomes ss.
- nþ becomes þþ (Noldorin ss).
- pn becomes mn. This is probably just historical, as inflected pn becomes mp.
- r after a consonant usually switches places before it.
- rȝ becomes ry.
- rs once became ss, but later Quenya has stable rs that evolved from rþ.
- sm inconsistently becomes mm, ss or zm (Noldorin rm).
- sr becomes rr.
- tm becomes nw.
- vn becomes mn.
- zl becomes ll.
- zr becomes rr.
- þr becomes þþ (Noldorin ss).
 Final consonants
When a cluster finds itself at the end of a word, an otherwise stable consonant or consonant cluster reduces even further.
- c(c) becomes t.
- ht becomes t.
- ld becomes l.
- ll becomes l.
- m(m) becomes n.
- nd becomes n.
- ng becomes n.
- nn becomes n.
- rn becomes r.
- rr becomes r.
- rt becomes r.
- ss becomes s.
- st becomes s.
- tt becomes t.
- x becomes s.
- z becomes s. Note that even after z becomes r among the Noldor, it remains s in forms where the z came at the end of a word.
- þ(þ) becomes s. Final þ does not seem to be allowed even among the Vanyar.
- qu or consonant forms that end with w are allowed to turn the qu into co or the w into o, which forms a distinct syllable and spares the consonant cluster from further simplification.
Though the above list is incomplete, it seems to describe a useful pattern that also implies:
- p(p) probably becomes t.
- Any cluster with s becomes s.
- Any cluster with r becomes r.
- Any cluster with l becomes l.
- Any cluster with n or m (besides rn and perhaps lm rm) becomes n.
Other ending mutation patterns are even less certain.
 Vowels in hiatus and unstable diphthongs
Many vowel combinations can exist in hiatus, but some may not. This is true whether the neighboring vowel is a short vowel, a long vowel or half of a diphthong pair.
- aa becomes ëa.
- ae becomes é, or e at the end of a word.
- ao becomes ó, or o at the end of a word.
- ee becomes ie.
- ei becomes í or i.
- ii becomes í or i. Note that even i-ei may be affected by this.
- oo becomes uo.
- ou becomes au or possibly ú.
- uu probably becomes ú or u.