Many irregularities affect the stem
Regular Spanish verbs split into three conjugation classes by their infinitive ending, -ar, -er, -ir, and each class has one fixed set of endings for every tense: hablar takes hablo, hablas, habla...; comer takes como, comes, come...; vivir takes vivo, vives, vive... Many irregularities affect the stem, but special paradigms can also change endings, as strong preterites do. The RAE distinguishes vowel alternations, consonant alternations and mixed changes, then also discusses suppletive roots and defective paradigms.
That classification runs to four working categories, plus one further pattern that isn't about changing forms at all: vocalic, consonantal, mixed, suppletive, and defective. Most English-language guides to Spanish irregular verbs skip this structure and default to a teaching-oriented split instead, stem-changing, spelling-change, irregular yo form, fully irregular, built around how a verb behaves in the present tense specifically. That split is a genuinely useful study aid, but it's a pedagogical shortcut rather than the actual grammatical mechanism, and it tends to fall apart once you follow one verb across more than one tense, which the sections below walk through directly. It's also not a coincidence that the verbs covered here skew toward the most frequently used verbs in the language, ir, ser, estar, tener, hacer, decir: Judy Hochberg's analysis of a Spanish frequency dictionary notes that its twelve highest-ranked verbs are irregular and discusses frequent use as one factor in the survival of irregular forms. The RAE also describes different formation strategies in other parts of Spanish morphology: Spanish superlatives covers the Latin-inherited -érrimo suffix and intensifying prefixes alongside -ísimo. These are additional patterns to learn, without implying that adjectives and irregular verbs share one mechanism.
Vocalic irregularities: the stem vowel itself moves
The largest irregular family swaps one stem vowel for another, sometimes for a full diphthong. E → ie covers verbs like acertar (acierto), entender (entiendo), and discernir (discierno). O → ue covers contar (cuento) and mover (muevo). Two much smaller patterns sit alongside these: i → ie has the basic models adquirir (adquiero) and inquirir (inquiero), while u → ue has jugar (juego) as its basic model. Derived verbs can inherit these patterns. In the present indicative, these diphthong patterns leave nosotros and vosotros unchanged because the stem is not stressed there: contamos and contáis keep the plain o, and only cuento, cuentas, cuenta, and cuentan carry the diphthong.
A related but distinct vocalic pattern swaps one plain vowel for another without ever forming a diphthong: pedir → pido, servir → sirvo, seguir → sigo. The RAE groups this e → i alternation under the same vocalic heading as the diphthong verbs above, because both are, mechanically, a vowel replacing a vowel in the stressed stem; the only difference is whether the replacement happens to land on a diphthong or a single vowel.
Consonantal irregularities: a sound gets swapped or added
A second RAE category covers verbs where a consonant, not a vowel, is what changes. The largest group by far is verbs ending in -acer, -ecer, -ocer, and -ucir, whose stem consonant becomes -zc- in the yo form and throughout the present subjunctive: conocer → conozco, aparecer → aparezco, conducir → conduzco, producir → produzco, traducir → traduzco, reducir → reduzco. This isn't quite every verb with those endings, though: hacer, decir, and satisfacer end in a vowel plus -cer/-cir like the -zco verbs above, but build their yo form a different way entirely (hago, digo, satisfago), and hacer's pattern is covered on its own just below. Mecer and remecer are a further outlier: they take a plain c → z swap (mezo, remezo), not the double -zc- the rest of the group uses. Other persons in their present indicative lack -zc-: conoces, conoce and conocemos. This says nothing about their irregularities in other tenses, such as conduje.
Hacer → haga and haber → haya illustrate consonant substitutions rather than the -zc- addition. These examples are not an exhaustive count of affected verbs: derivatives of hacer can preserve its alternation, as rehacer → rehaga does.
Mixed irregularities: vowel and consonant change together
A smaller group of verbs does both at once. Decir is the clearest example: its yo form digo combines e → i with c → g. The forms dices, dice and dicen also have i, while decimos and decís retain e. Decir and digo each have two syllables; the change is not a syllable being dropped. The RAE treats this as its own category, mixta, precisely because neither the vocalic pattern nor the consonantal pattern alone explains digo; only both, acting on the same stem together, do.
Whatever irregularity shows up in a verb's yo form tends to carry into every person of the present subjunctive, not just yo. Decir's irregular digo becomes diga, digas, diga, digamos, digáis, digan, the same way conocer's conozco becomes conozca, conozcas, and so on: this yo-based study rule works for those models. Special paradigms require separate forms, such as soy → sea, he → haya, sé → sepa and voy → vaya.
Special conjugations and suppletive roots
The RAE discusses haber, ser, ir, estar and dar under special conjugation, then distinguishes suppletive roots. Ir and ser illustrate suppletion through sharply different stems: ir has voy, iba and fui, while ser has soy, era and fui. Vaya belongs with the present stem of ir, not with fui. Listing five special conjugations does not mean that all five combine different Latin verbs.
Estar is the mildest case in this group: it conjugates almost entirely by the regular rules for an -ar verb, with irregular forms including estoy, esté and the strong preterite estuve, covered along with the rest of Spanish's strong preterites in the irregular preterite verbs guide. Dar is the reverse oddity: it's an -ar verb by form, but its preterite (di, dio, dieron) borrows -er/-ir endings even though the infinitive would suggest -ar ones. For ser's complete paradigm across every mood and tense, see the full ser conjugation table.
Defective verbs: irregular by missing forms, not by changing them
The RAE also discusses a separate pattern that isn't about alternation at all: verbs that simply lack certain forms of their own paradigm. Weather verbs like llover and nevar are normally impersonal and third-person singular in their weather senses, but figurative uses can have other subjects. Soler is most often used in the present and imperfect, in both indicative and subjunctive; the RAE also records rare compound forms such as he solido. It is not used in the future, conditional or imperative. A third group includes atañer and concernir ("to concern" or "to be relevant to"). The RAE describes these as third-person verbs because their subjects are events or propositional contents rather than people.
The "stem-changing vs. fully irregular" split you'll see elsewhere is a study aid, not this classification
If you've studied Spanish through an English-language textbook or app, the categories above probably don't match the ones you learned. Most beginner materials sort irregular verbs into "stem-changing," "spelling-change," "irregular yo form," and "fully irregular," or some near variant, because that grouping maps directly onto which forms a learner needs to drill for the present tense. It's a genuinely useful study aid, and it isn't wrong exactly, but it's organized around memorization difficulty in one tense, not around the underlying mechanism. Tener, for instance, gets called "stem-changing" in most textbooks because of tengo and tiene, but it also shortens its stem in the future (tendré) and takes a u-stem preterite (tuve) that has nothing to do with either of those present-tense patterns; no single textbook label covers all three at once.
For that reason, this page groups verbs by what the RAE's grammar says is actually happening to the stem, rather than by which tense a learner encounters first. If you want the present-tense entry point mapped across specific tenses, the six-tense overview chart walks through where each one sits, and the u-stem, i-stem, and j-stem families cover the separate stem groups that only surface in the preterite.
Three mix-ups these categories help explain
Conosco is a common misspelling of conozco; verbs following the conocer model require -zc- in the yo form and present subjunctive. The exceptions described above follow other models.
Learners sometimes assume hacer's subjunctive follows the same -zco pattern as conocer, expecting something built the same way. It doesn't: haga uses a consonant substitution, not the conocer model. The same alternation occurs in derivatives such as rehacer → rehaga.
Calling tener simply "irregular" without specifying which tense obscures a real distinction: tengo (present, a consonantal yo-form irregularity) and tuve (preterite, part of the u-stem family) come from two separate mechanisms wearing the same infinitive, not one irregularity showing up twice.