It may just be that I'm demonstrating my gross ignorance, but I can't seem to find a 'why' for stem-changing verbs in Spanish. I understand that there is some sort of perceived weakness in the vowel that is emphasized by the addition of stress to the syllable, and that the vowel is somehow strengthened by the shift. I just don't understand the governing principles of that shift. Why do some vowels shift and not others? What is it that says that the 'e' or 'o' in this verb will change, but not the 'e' or 'o' in that verb?

Every search I've done just comes back with a lot of people saying 'that's just how it is, memorize it', and that's not really an answer. Any help out there?

  • The change has to do with the Yod sound of some verbs.. The yod affects the preceiding vowel... – user9437 Mar 27 '15 at 1:56

The diphthongization of front and back mid vowels that's referred to here is an historical process moving from Classical Latin to Vulgar Latin to Castilian Spanish, over about a millennium.

This is the way it works:

  • Classical Latin (ca 0 CE) had long and short vowels.
  • Vulgar Latin (ca 0-1500 CE) lost the Classical vowel length distinction.
  • Vulgar Latin innovated a new open/closed distinction for mid vowels e and o.
  • Mid vowels that were short in Classical Latin became open in Vulgar Latin.
  • Castilian Spanish (ca 1200 CE) changed Vulgar Latin open e [ɛ] to ie, and open o [ɔ] to ue.

Here's what it says in the Mexican standard high school textbook on Greek and Latin etymology of Spanish (Mateos M, A, Etimologīas Grecolatinas del Espanol, Editorial Esfinge, Mexico DF):

(51) La e breve y el diptongo ae del latín clásico se convirtieron en la e abierta del latín vulgar y se transformaron generalment in ie, al pasar al castellano.

(54) La o breve del latín clásico se convirtieron en la o abierta del latín vulgar y se diptongó in ue (pasando por uo), por regla general.

In essence, if the mid vowel was short in Classical Latin, it's likely to be a diphthong in Spanish now

  • Ahh, the missing link: LATIN. With a heavy dose of historical phonology. Thank you. I was hoping to find something that would make it easier to explain to my young HS minds, but I doubt they will benefit from this knowledge. I, on the other hand, am deeply grateful. – jrob Apr 18 '13 at 19:24
  • It doesn't seem to bother Mexican HS minds; Mexico requires everyone to pass a standardized exam on this book to get into college. – jlawler Apr 18 '13 at 20:23
  • You might also find this useful; it's the handout from a talk I gave on historical linguistics at a science fiction conference. There's nothing in it -- or in linguistics in general -- that's beyond high school minds. That's where there rest of the first world teaches and learns it, just like trigonometry and chemistry. – jlawler Apr 18 '13 at 23:08

The 'governing principle' is stress. In Spanish, [ɛ] and [ɔ] become diphthongs in stressed positions, explaining why niego, niegas, niega, niegan have diphthongs, while negamos does not (the stressed penultimate is [a] there).

See the Wiki article on vowel breaking for other examples of diphthongisation in stressed syllables.

  • The stress aspect of it I understand, but if it is simply the presence of the 'e' or the 'o', then why don't 'beber' or 'comer' or 'colocar' or any of dozens (hundreds?) of verbs with those same sounds have that same stem-change? There is something more to it than just stress, some sort of interaction with other sounds in the word that I can't see or have some sort of mental block towards... – jrob Apr 18 '13 at 15:21
  • your answer was not lacking, it was my lack of understanding of what was meant by IPA vowels you mentioned. With the other answer, I now see what you were trying to indicate. Thank you for adding to my understanding. – jrob Apr 18 '13 at 19:27
  • 1
    @jrob If it's like Portuguese, the answer is because of vowel harmony. In words with medium height root vowels (o and e in the root) and low theme vowel (-ar), the root vowel will try to match the height of the theme vowel. The same is true in verbs derived from long theme vowels in Latin Therefore you have ciego from cegar, tiene from tener (which is tenēre with a long ē), but como from comer (which is from comedere with a short e). siento from sentir from sentīre, BUT beber is from bibere which contains a short theme vowel, thus no vowel harmony. – Ryan Ward Jun 13 '14 at 15:45

One last thing. Vulgar latin, like Italian and Catalan, only had the distinction of ɛ-e and ɔ-o in stressed syllables. They were reduced (ie. merged) to e and o in unstressed syll.

So, the alternation 'mover' - 'muevo' comes from Vulgar latin /mo'vere/ - /'mɔ.vo/. The stem, /mɔv/, had been reduced to /mov/ in unstressed positions, so it wasn't diphthongized.


The answer is because of vowel harmony in Vulgar Latin. The rule is not 100% anymore because Vulgar Latin was spoken so long ago. Clearly since the paradigm started, the stem changing has been applied to verbs that don't match the pattern, in general it is as follows:

In verbs with medium height root vowels:

  • e: cegar, sentir, cerrar, mentir
  • o: dormir, mover, entender, volar

If those verbs had a long (or doubled) vowel in Latin as their theme vowel then they were most likely going to be pronounced with a wide e - ɛ or a wide o - ɔ in conjugations where they retain the tonic stress. In the following examples I'll use first person singular i.e. yo

Eventually those wide mid vowels became ie and ue in Spanish, because Spanish phonology does not contain wide mid vowels.

Some simplifications are going to be made here, but here are some etymological timelines:

  • caecāre long theme vowel -āre -> cegar: yo cego -> cɛgo -> ciego
  • sentīre ->sentir: yo sento -> sɛnto -> siento
  • serāre -> cerrar: yo cerro -> cɛrro -> cierro
  • dormīre -> dormir: yo dormo -> dɔrmo -> duermo
  • volāre -> volar: yo volo -> vɔlo -> vuelo

However the following verbs are not subject to the rule because the theme verbs are short, marked with a caret.

poner (ponĕre), cocer (coquĕre), beber (bibĕre), meter (mittĕre), romper (rumpĕre)

  • 1
    But cocer does stem change :-) Interestingly, the pattern isn't perfect, as there are discrepancies between Spanish and other Iberian languages as to which verbs stem change (Asturian stem changes correr and responder for instance) – user0721090601 Oct 12 '14 at 23:15

It's basically for the sound. Without stem-changes, certain words would sound awkward would be harder to say in a sentence, and the sentence would not flow smoothly. I don't know about stress on vowels, though. Comer and Beber aren't conjugated is because they sound fine.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.