O-to-U Stem-Changing Verbs

Parallel to the e-to-i class, European Portuguese has an o-to-u stem-changing class for -ir verbs whose stem contains the vowel o. The stem raises o to u in exactly the same two places: the first-person singular of the present indicative, and throughout the present subjunctive. Everything else about these verbs is regular. The most important members are dormir, cobrir, descobrir, engolir and tossir — along with one oddball that everyone learns separately, subir, whose alternation is the other way round.

The core pattern

Remove the -ir ending and look at the stem vowel. If the stem is dorm-, cobr-, descobr-, engol-, toss-, consum-, or a few others, the o raises to u in two environments:

• In the 1sg present indicative (always).
• In every person of the present subjunctive.

Every other form preserves the original o. No preterite forms, no imperfect, no future or conditional are affected. The raising is exactly the same phonological process as e → i: the unstressed subjunctive ending -a and the indicative -o pull a preceding mid-back vowel up one step. In e verbs the step is e → i; in o verbs it is o → u.

Dormir (to sleep) — full paradigm

The pair durmo / durma is spelled identically except for the ending; the u is present in both because both endings (-o and -a) trigger the raising. In dormes, dorme, dormimos, dormem, the stress either falls on a different vowel or the ending begins with a non-raising segment, and the o stays.

Cobrir (to cover) and descobrir (to discover / to find out)

Descobrir is the most common compound and follows exactly the same pattern (eu form descubro, subjunctive descubra, descubras...):

Other compounds behave identically: encobrir (to cover up / to conceal — encubro, subj. encubra), recobrir (to re-cover — recubro, subj. recubra).

Engolir (to swallow)

Engolir is also used figuratively for putting up with something unpleasant: engolir um sapo (literally "to swallow a toad") means to bite one's tongue or to accept an insult without retaliating.

Tossir (to cough)

Consumir and sumir

Consumir (to consume) behaves as you would expect by analogy — eu form consumo — but because its infinitive already contains a u, there is nothing to raise. No alternation is visible on the surface. Sumir (to disappear) looks similar at first glance, but it actually belongs to a different micro-class: sumo, somes, some, sumimos, somem. That is the inverse alternation pattern shared with subir, treated in the next section. So keep consumir as regular-looking (no surface alternation) and treat sumir together with subir.

The odd case of subir

Subir (to go up / to climb) does something different from everything above, and it is important to flag it clearly. Its infinitive already contains u — so a pure raising rule would produce no alternation. Instead, the stressed non-eu forms lower the u to o, while the 1sg stays with u. In other words, subir alternates u ↔ o, going the opposite direction from the raising class. The result looks visually similar to what you would expect from the boot pattern, but the rule is reversed.

The same behaviour applies to a short list of verbs that etymologically already had a high vowel: fugir (to flee — fujo, foges, foge, fugimos, fogem; subj. fuja, fujas, fuja, fujamos, fujam), sacudir (to shake — sacudo, sacodes, sacode, sacudimos, sacodem), sumir (to disappear — sumo, somes, some, sumimos, somem). These are the same verbs that raise spelling flags in any careful European Portuguese textbook: they invert the expected direction of the alternation.

The imperative

As with every -ir verb in this family, the imperative inherits the asymmetry between the unchanged indicative stem and the changed subjunctive stem:

• Affirmative tu = the 3rd-person singular present indicative (no raised vowel): dorme!, cobre!, engole!, tosse! — and for subir: sobe!.
• Affirmative você / vocês and every negative imperative = the corresponding present subjunctive: durma!, cubra!, engula!, tussa!, suba! — and não durmas!, não cubras!, não engulas!, não tussas!, não subas!.

How this compares to Spanish

Spanish splits the back-vowel -ir verbs into two subclasses (o → ue in dormir, morir; u → u elsewhere). Portuguese simplifies the picture: all of these verbs raise o straight to u — no diphthong, no ue.

The most striking contrast is subir itself: Spanish subir has no alternation at all (subo, subes, sube, subimos, suben), while Portuguese subir alternates. Do not import the Spanish paradigm here.

English-speakers' angle

English speakers learning Portuguese often overcorrect. They hear "stem change" and think every occurrence of o should become u, or they try to apply the boot pattern of dormir to subir — but subir flips the expected direction. The cleanest way to handle the class is to learn each verb's 1sg present indicative together with its present subjunctive eu form as a pair, and then predict the rest. For dormir: durmo / durma. For cobrir: cubro / cubra. For subir: subo / suba (both with u, and then remember that tu, ele, eles will be sobes, sobe, sobem).

Common Mistakes

Key takeaways

Two patterns, mirror-image in direction:

1. Raising class (dormir, cobrir, descobrir, engolir, tossir): o → u in the 1sg present indicative and the entire present subjunctive; o everywhere else in the present. No other tenses affected.
2. Alternating class (subir, fugir, sacudir, sumir): keep u in the 1sg present indicative and the entire present subjunctive; drop to o in the stressed tu, ele, eles present indicative forms.

The subjunctive is always predictable from the 1sg indicative — whatever the 1sg does, the subjunctive extends to all persons. That single insight unlocks the entire class.