Some Brazilian Portuguese verbs keep their endings perfectly regular but quietly change the vowel inside the stem when you conjugate them. Prefiro but preferir; durmo but dormir. This page maps out the three main stem-change patterns, explains the logic behind each one, and — crucially — separates real stem changes from the spelling changes that only look like changes on paper.
The big distinction: stem change vs. spelling change
Before anything else, internalize this difference, because mixing the two is the single most common confusion for learners.
- A stem change alters both the sound and the spelling. When you say durmo instead of dormo, you actually pronounce a different vowel. The change is audible.
- A spelling change alters only the spelling to keep the sound the same. When you write fiquei instead of the expected ficei, the /k/ sound never changed; the spelling adjusted to protect it.
English has nothing quite like either, but stem changes will feel vaguely familiar: English "sing → sang → sung" changes an internal vowel too. The difference is that Portuguese stem changes are predictable by person, not just by tense, and they cluster in specific, learnable groups.
Pattern 1: -ir verbs with e → i
A group of -ir verbs raises the stem vowel e to i in exactly two places: the eu form of the present indicative, and every form of the present subjunctive. Everywhere else, the e stays put.
Take preferir (to prefer):
| Form | Present indicative | Present subjunctive |
|---|---|---|
| eu | prefiro | prefira |
| você / ele / ela | prefere | prefira |
| nós | preferimos | prefiramos |
| vocês / eles / elas | preferem | prefiram |
The logic is phonological: the eu present form and the whole subjunctive are built on the same underlying stem, so they share the raised vowel. The other present forms keep the original infinitive vowel.
Verbs in this group include preferir, sentir (to feel), servir (to serve), repetir (to repeat), vestir (to dress), and mentir (to lie).
Eu prefiro café sem açúcar, mas ela prefere bem doce.
I prefer coffee without sugar, but she prefers it really sweet.
Sinto muito, eu não sabia que era hoje.
I'm really sorry, I didn't know it was today.
Aqui no restaurante a gente serve almoço até as três.
Here at the restaurant we serve lunch until three.
Notice in the first example how prefiro (eu) and prefere (ela) sit side by side — the vowel genuinely changes, and you hear it.
Pattern 2: -ir verbs with o → u
A parallel group of -ir verbs raises o to u in the same two spots: the eu present and all of the present subjunctive.
Take dormir (to sleep):
| Form | Present indicative | Present subjunctive |
|---|---|---|
| eu | durmo | durma |
| você / ele / ela | dorme | durma |
| nós | dormimos | durmamos |
| vocês / eles / elas | dormem | durmam |
Other verbs here include cobrir (to cover), descobrir (to discover), tossir (to cough), and engolir (to swallow).
Eu durmo muito mal quando faz calor.
I sleep really badly when it's hot.
O bebê dorme a tarde inteira, que sorte!
The baby sleeps the whole afternoon, what luck!
Acho que descubro a senha se eu pensar um pouco.
I think I'll figure out the password if I think a bit.
It is the same machinery as Pattern 1, just with a different pair of vowels. If you learn dormir → durmo, you already understand engolir → engulo and cobrir → cubro.
Pattern 3: -er verbs where the vowel opens or closes (sound only)
Here is where Brazilian Portuguese hides a change that English speakers almost never notice and that does not show up in spelling. Many -er verbs (and the verb poder) shift the quality of the stem vowel — open /ɔ/ vs. closed /o/, or open /ɛ/ vs. closed /e/ — depending on the form, even though the letter on the page stays the same.
Take poder (to be able to). It also has an irregular eu form, but watch the vowel quality in the rest:
| Form | Spelling | Stressed vowel sound |
|---|---|---|
| eu | posso | closed /o/ |
| você / ele / ela | pode | open /ɔ/ |
| nós | podemos | (unstressed) |
| vocês / eles / elas | podem | open /ɔ/ |
The o in posso and the o in pode are written identically but pronounced differently — closed in posso, open in pode. Brazilians do this automatically; for a learner it is something you absorb by listening, not by reading.
Eu posso te ajudar agora, se você puder esperar dois minutos.
I can help you now, if you can wait two minutes.
A gente pode ir de carro ou a pé, tanto faz.
We can go by car or on foot, doesn't matter.
Don't confuse these with spelling-change verbs
Verbs like ficar → fiquei, chegar → cheguei, and começar → começo look like they're changing too — but they aren't changing their sound. The /k/, /g/, and /s/ stay exactly the same; only the letters shift to obey Brazilian spelling rules. Those belong on the spelling-change verbs page, and you should keep them in a completely separate mental box.
Eu cheguei cedo e fiquei esperando na porta.
I arrived early and stayed waiting at the door.
Here cheguei and fiquei are spelling changes (the gu and qu protect the hard sound). Contrast that with durmo above, where the vowel genuinely shifts.
A note for Spanish speakers
If you already speak Spanish, unlearn some of your reflexes here. Spanish has many stem-change classes — e→ie (pensar → pienso), o→ue (dormir → duermo), e→i (pedir → pido) — and they spread across all three conjugations. Brazilian Portuguese has far fewer: there is no e→ie diphthong (Portuguese prefiro, not prefiero; sinto, not siento) and no o→ue diphthong (durmo, not duermo; posso, not puedo). Do not import the Spanish diphthongs — they will sound wrong every time.
Eu sinto frio à noite.
I feel cold at night.
Eu não posso ir hoje.
I can't go today.
Common Mistakes
❌ Eu prefero chá.
Incorrect — the eu form raises e to i: it must be prefiro.
✅ Eu prefiro chá.
I prefer tea.
❌ Eu dormo até tarde no domingo.
Incorrect — the eu form of dormir raises o to u: durmo.
✅ Eu durmo até tarde no domingo.
I sleep in late on Sunday.
❌ Eu siento muito.
Incorrect — this is the Spanish diphthong; Portuguese has no e→ie change.
✅ Eu sinto muito.
I'm very sorry.
❌ Ela prefire viajar de ônibus.
Incorrect — only the eu form raises the vowel; the ela form keeps e: prefere.
✅ Ela prefere viajar de ônibus.
She prefers to travel by bus.
❌ Eu não pudo ir.
Incorrect — pudo is the preterite; the present eu form of poder is posso.
✅ Eu não posso ir.
I can't go.
Key Takeaways
- Patterns 1 and 2 (e→i, o→u in -ir verbs) change the vowel in the eu present and the whole present subjunctive only. Sound and spelling both change.
- Pattern 3 (-er verbs and poder) changes only the sound (open vs. closed vowel); the spelling never moves. Learn it by ear.
- Spelling-change verbs (ficar, chegar, começar) are not on this page — their sound never changes, only their letters.
- Spanish speakers: drop the diphthongs. Portuguese says prefiro / sinto / durmo / posso, never prefiero / siento / duermo / puedo.
Now practice Portuguese
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Start learning Portuguese→Related Topics
- Spelling-Change VerbsA2 — Verbs that change spelling — but not sound — to protect a consonant's pronunciation across the conjugation.
- Stem-Changing -ir VerbsA2 — The predictable e→i and o→u vowel shift in the eu form of many Brazilian Portuguese -ir verbs, and why it reappears throughout the subjunctive.
- Present Indicative: Regular -ir VerbsA1 — How to conjugate regular -ir verbs in the Brazilian Portuguese present indicative, and why they differ from -er verbs in only one form.
- Present Indicative of PoderA1 — How to conjugate poder (can, may, be able to) in the Brazilian Portuguese present, the three meanings it covers, and the everyday 'pode ser'.