Quenya does not have mutations in its grammar, like Sindarin does, but adding an ending to a root tends to cause a slight change in that root (except for in verbs which have their own variations within their conjugations). Therefore, these work for nouns, adjectives, and etc.
Before any ending, long root vowels become shortened:
á > a
é > e
í > i
ó > o
ú > u
In two-syllable words containing identical vowels, the second vowel dissappears before an ending:
Ex. "feren" has a plural of "ferni", "laman" has a plural of "lamni"
When the plural ending -r or the pronoun ending -nya are added, an end -ë becomes -e-
Ex. "lantanë" has a plural of "lantaner"; "órë" + "-nya" becomes "órenya
When other endings are added, an end -ë becomes -i-
When any ending is added, an end -o becomes -u-
When any ending is added, a last-syllable -k or -k- becomes -qu-