Why is it "pronunciation" and not "pronounciation"?
Generally speaking, when adding a suffix to a word in English, while the last letter(s) may undergo changes to accommodate the addition, the rest of the word is left unchanged. As examples in that sentence alone – general/generally, speak/speaking, add/adding/addition, change/changes/unchanged.
The only exception off the top of my head is the word pronounce. Nearly all of its variations — pronouncing, pronounced, pronounceable, pronouncement — all maintain the spelling of the word, save for the final e. However, one of its noun forms is pronunciation, where the central ⟨ou⟩ vowel is swapped out for a ⟨u⟩. Is there a historical reason why specifically the word pronunciation has its central vowel changed, among all of the forms of the word pronounce? Are there any other words in English which exhibit this phenomenon for the same reason?