Currently, I am learning Portuguese. I have some knowledge of Spanish as well.

The biggest difference in conjugation (indicative present tense) that I have found between Spanish and Portuguese is that Portuguese does not go through stem change (at least not that often) than Spanish. In Spanish, it is very usual that we find that we have to change ei, oue, etc. When we conjugate, however, Portuguese does not have it, except in some cases. Why is that ?

Even though Portuguese does not go through stem changes so often as Spanish does, there are still cases when it happen, especially in first person singular (Eu). In some cases, the change is only to preserve the pronunciation of the infinitive form (Ex: FugirEu fujo). While in other cases, it kind of works like Spanish (Ex: PreferirEu prefiro; but você prefere, DormirEu durmo; but você dorme).

So why in these cases, there will be stem changes in Portuguese but not for another pronouns. Is there a reason behind?

  • Note that "você prefere" is Brazilian Portuguese. In Portuguese proper, it would be "tu preferes". Dec 16, 2022 at 13:54

2 Answers 2


In short, it depends on the different phonological processes each language had in its history and the regularizations that may or may not have happened in their words and paradigms.

The past tense of many verbs like "fazer" (fiz) or "saber" (soube) has irregular vowels in the root for reasons that trace back to latin and beyond.

The reason why "dormir" changes in the present is because of a historical process of metaphony triggered by the theme vowel "i". So it happens with all the verbs of the third conjugation! o > u (as in "dormir"); e > i (as in "seguir"). So this theme vowel disappeared and left a [+ high] trace in the vowel in the root, that's why it is not "dormio" anymore, although you can find this form in medieval galician-portuguese texts!:

E por esto nõ dormio eu, porque nõ poss'eu coita dar a quẽ me sẽmpre coita deu.

Translation by me:

E por isto não durmo eu, porque não posso eu dar sofrimento a quem sempre me deu sofrimento.

And as you can see in words like "esto" that became modern "isto", Portuguese has many cases of metaphony all over it, something that didn't happen in Spanish.


In Spanish, it is very usual that we find that we have to change e ⟶ i, o ⟶ ue, etc. When we conjugate, however, Portuguese does not have it, except in some cases. Why is that?

Ergative Man has touched on the vowel raising, so I'll focus on the diphthongs.

In the ancestor of Spanish and Portuguese, there were seven vowels, rather than five: i, e, ɛ, a, ɔ, o, u.

In Modern Spanish, there are only five. ɛ and ɔ disappeared: they turned into ie and ue when stressed, but e and o when unstressed. So sometimes, when a word contained ɛ or ɔ, and it's sometimes stressed and sometimes unstressed—you get a vowel alternation.

In (most dialects of) Portuguese, ɛ and ɔ turned into e and o when unstressed, but stuck around when stressed. This means that you get an alternation between o and ɔ rather than o and ue. The Portuguese alphabet doesn't consistently distinguish between these sounds (there are accent marks but they aren't universal), so the alternation is a lot less visible. But it's still there: móro "I remain" has ɔ, for example, while morámos "we remain" has o.

  • 1
    Moramos (and also morámos) actually has [u], since unstressed |o| becomes /u/, at least in European Portuguese. Dec 15, 2022 at 2:48
  • @JanusBahsJacquet I admit most of my experience is with Brazilian; a lot of my hedges about dialect differences are because I actually don't know much about them! I vaguely recall there are some that have lost the o~ɔ distinction entirely, for example.
    – Draconis
    Dec 15, 2022 at 3:26

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