Stem-Changing Verbs Overview

Some Portuguese verbs look perfectly regular in their infinitive but change their stem vowel when conjugated. The endings stay the same as any regular verb in their class — it is only the vowel inside the stem that shifts. These are called stem-changing verbs, and they follow predictable patterns once you know what to look for.

Why stems change

Portuguese vowels behave differently depending on whether they carry stress. When the stress falls directly on the stem, a vowel may open or close compared to its unstressed form in the infinitive. This is not random irregularity — it is a natural phonological pattern of the language. The verb endings remain entirely regular; only the internal vowel moves.

E to I changes (-ir verbs)

The most important stem-change pattern in Portuguese. In certain -ir verbs, the e in the stem becomes i in the first person singular (eu) of the present indicative. The remaining forms keep the original e.

Here is sentir (to feel) in the present indicative:

Common verbs that follow this pattern:

Note that seguir also changes the consonant (sigo, not seguo) — this is a spelling adjustment on top of the vowel change.

O to U changes (-ir verbs)

A parallel pattern: in certain -ir verbs, the o in the stem becomes u in the first person singular of the present indicative.

Here is dormir (to sleep) in the present indicative:

Common verbs with this pattern: cobrir (cubro), descobrir (descubro), tossir (tusso), engolir (engulo).

A related but inverse pattern affects subir (to go up) and a few other -ir verbs. Here the infinitive already has u, so the eu form keeps the u (subo) while the stressed non-eu forms switch to o: subo, sobes, sobe, subimos, sobem. The same applies to fugir (fujo, foges, foge, fugimos, fogem) and sacudir (sacudo, sacodes, sacode, sacudimos, sacodem).

E to EI changes (-ear verbs)

Verbs ending in -ear insert an i before the ending whenever the stress falls on the stem. This affects the eu, tu, ele, and eles forms.

The model verb is passear (to stroll): passeio, passeias, passeia, passeamos, passeiam. Other verbs following this pattern include recear (to fear), cear (to dine), and nomear (to name, to appoint).

Closed and open vowel alternation

This is a subtler change that affects pronunciation more than spelling. In some -er and -ir verbs, the stem vowel is closed (like the e in "they") in the infinitive and the nós form, but open (like the e in "bet") in the tu, ele, and eles forms.

Take dever (to owe, must):

• eu devo — closed [e]
• tu deves — open [E]
• ele deve — open [E]
• nós devemos — closed [e]
• eles devem — open [E]

This open/closed distinction is not always shown in spelling, but it is essential for natural European Portuguese pronunciation. Other verbs with this alternation include poder (posso, podes [O], pode [O], podemos, podem [O]) and mover (movo, moves [O], move [O], movemos, movem [O]).

The boot pattern

Many stem changes follow what is sometimes called a boot pattern (or shoe pattern). Imagine drawing a line around the conjugation table: the change occurs in the forms where the stem carries the stress — eu, tu, ele/ela/você, eles/elas/vocês — and does not occur where the ending carries the stress — nós (and the rare vós). The affected forms, when circled on a chart, form the shape of a boot.

This pattern holds across several Romance languages. In Portuguese, it is most visible in the -ear verbs (passeio, passeias, passeia, passeamos, passeiam) and in the open/closed vowel alternation.

For the e-to-i and o-to-u changes in -ir verbs, the boot narrows further: only the eu form changes in the present indicative, while the present subjunctive extends the changed vowel to all forms (sinta, sintas, sinta, sintamos, sintam).

Summary of the most common patterns

How to learn them

Group stem-changing verbs by pattern rather than trying to memorise each one individually. The e-to-i and o-to-u changes in -ir verbs are the highest priority — these verbs are extremely common in daily conversation. Start with sentir, dormir, servir, and vestir, and you will already cover a large share of the stem-changing verbs you encounter.

When you meet a new -ir verb with e or o in the stem, check whether the eu form changes — it almost certainly does. From there, the present indicative and present subjunctive pages will show you exactly how these changes play out across the full conjugation.