I know that vulgar latin experienced a diphthongization with several of the vowels when in tonic positions of a word.
My specific question is why did all latin originating words not consistantly do that?
When saying "I close", why did the language evolve to think "Yo cerro" needs to be changed to "Yo cierro", but when saying "hill", literally saying "cerro" is fine? Both words are stressed on the first syllable.
I looked up the etymology and they both derive from latin, so shouldn't they experience the same linguistic evolution?