Spanish verbs are a complex area of Spanish grammar, with many combinations of tenses, aspects and moods (up to fifty conjugated forms per verb). Although conjugation rules are relatively straightforward, a large number of verbs are irregular. Among these, some fall into more-or-less defined deviant patterns, whereas others are uniquely irregular. This article summarizes the common irregular patterns. As in all Romance languages, many irregularities in Spanish verbs can be retraced to Latin grammar.

Orthographic changes

Due to the rules of Spanish orthography, some predictable changes are needed to keep the same consonant sound before a or o and e or i, but these are not usually considered irregularities. The following examples use the first person plural of the present subjunctive: Other predictable changes involve stress marks, i—y alternations and i-dropping, some of which are sometimes considered as irregularities. The examples are several forms of otherwise regular preterites:

Stem-vowel changes

There are two kinds of changes that can affect stem vowels of some Spanish verbs: diphthongization and vowel raising. Both changes affect -e- or -o- in the last (or only) syllable of a verb stem. Diphthongization changes -e- to -ie-, and -o- to -ue-. Vowel raising changes the mid vowels -e- and -o- to the corresponding high vowels: -i- and -u- respectively. Some verbs, in their various forms, can exhibit both kinds of changes (e.g., , (e-ie-i**ir); , , ) (o-ue-u).

Diphthongization

Some verbs with -e- or -o- in their stem are inherently diphthongizing, whereas others are not: their identities must be learned individually. In a diphthongizing verb, the change turns -e- into -ie- and -o- into -ue- when the syllable in question is stressed, which in effect happens only in the singular persons and third-person plural of the present indicative and present subjunctive, and in the imperative (all other tenses and forms are stressed on their endings, not their stems). The dictionary form always has the vowel, not the diphthong, because, in the infinitive form, the stress is on the ending, not the stem. Exceptionally, the -u- of (u-ue -gar, -jugar) and the -i- of and (i-ie) also are subject to diphthongization (', etc.; ', etc.). In word-initial position, ie- is written ye- (' > ') (e-ie > ye) and ue- is written hue- (' > '****) (o-ue > hue, oler). Also, the -ue- diphthong is written -üe- after g, with the diaeresis to indicate that the letter is not silent (' > ') (reflexive, go-güe -zar). The following examples show that all three conjugations (-ar, -er, and -ir verbs) include some diphthongizing verbs (only some tenses and persons are shown, for contrasting purposes):

Present indicative

Present subjunctive

(*) In Central America pensés, contés, etc. are used, but Spanish Royal Academy prescribes pienses, cuentes, etc., according to Rioplatense Spanish.

Imperative

The verbs sentir and dormir also undergo vowel raising. Additional diphthongizing verbs include ' (o-ue), ' (e-ie), ' (o-ue), ' (-zar e-ie, -ezar), ' (o-ue), ' (e-ie), ' (o-ue), ' (o-ue, -morir), ' (o-ue), ' (o-ue), ' (o-ue, -poder), ' (o-ue), ' (e-ie, -querer), ' (o-ue), ' (e-ie-i), ' (e-ie, -tener, G-Verb), ' (e-ie, -venir, G-Verb), ' (o-ue), and (o-ue, -olver). Many verbs with -e- or -o- in the root do not alternate. Common non-diphthongizing verbs include ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', ', and '. Less frequent verbs of this kind are often a source of mistakes for children learning to speak, and also for some adults:

Vowel raising

Vowel raising appears only in verbs of the third conjugation (-ir verbs), and in this group it affects, , (alternative of the more common ) and nearly all verbs which have -e- as their last stem vowel (e.g. , ); exceptions include ', ' and (all three diphthongizing, e-ie).

Affected forms

The forms that exhibit the change can be described negatively as those in which the stem vowel is not diphthongized and the ending does not contain stressed i or the -ir- sequence. In other words, vowel raising affects the forms whose endings do not contain an i which is not part of a diphthong, taking into account that diphthongizing overrides vowel raising. In effect, for diphthongizing verbs (e.g. ', '), the vowel-raising forms are: For non-diphthongizing verbs (e.g. ') it affects these same forms (pidamos, pidáis, pidiendo, pidió, pidieron, pi'diera''...), plus: The forms which do not undergo either diphthongizing or vowel raising are:

Affected verbs

Verbs which are diphthongizing and vowel-raising include: The diphthongizing -er verb ' exceptionally undergoes vowel rising in the gerund, but the first- and second-person plural of the present subjunctive are regular (podamos, po'dáis''). Non-diphthongizing vowel-rising verbs include: The vowel-raising verb is usually diphthongizing (with ye- forms as yergo...), not-diphthongizing forms are however valid but rare (irgo...) (e-ie > ye-i -guir, -erguir).

Diphthongs and hiatus

Cambio vs. envío

Diphthongs in the infinitive may be preserved throughout the conjugation or broken in the forms which are stressed on the stem, depending on whether the i or u in contact with a/e/o take the stress or not. The stressed vowel is marked bold in the examples: ' > cambio, but ' > envío (requiring an acute accent to indicate the resulting hiatus). The Real Academia Española does not consider either behaviour as irregular, but illustrates each with six "regular" models, one for each possible diphthong in the infinitive: ', ', ', ', ' and ' for diphthong-keeping verbs and ', ', ', ', ' and ' for diphthong-breaking ones. Remember that the presence of a silent h does not break a diphthong, so a written accent is needed anyway in rehúso. All verbs ending in -guar are diphthong-keeping, as well as ', ', ' and '. Two diphthongs are kept in ' > desa'hucio (again the -h- makes no difference), which thus follows both the anunciar and causar'' models. Diphthong-breaking verbs include ', ', ', ', ', ', ', ', ', ', ', '. The verbs ', ', ', ' and ' are also diphthong-breaking (crío, guí'e), but when the stress falls on the endings the resulting forms are generally considered as monosyllables and thus written without accent: crie, fie, guiais, lieis...''. In spite of that, the regular accentuation rules can also be used if they are pronounced as bisyllabic: crié, guiáis.... For the verbs ' and ' both options are valid: adecuo or adecúo. The ui diphthong in ' is kept throughout the conjugation despite the fact of the i getting the stress in forms such as cui'do (written without'' stress mark).

Verbs ending in -uir and -oír

All verbs ending in -uir (e.g. ', ', ') add a medial -y- before all endings not starting with i: construyo, construyes, construy'a... Taking into account that these verbs also undergo the change of unstressed intervocalic i to y (see orthographic changes above), they have many forms containing y''. This also applies to the forms of ' and ' that do not undergo the -ig- change: oyes, oye, oyen Some regular forms of ', ' and are written without stress mark if considered monosyllabic, but may bear it if pronounced as bisyllabic: vosotros huis or huís (present), yo hui or huí (preterite). Logically, loses the diaeresis before y: arguyo, arguyó (gü-gu, -güir)...

Other common irregular patterns

Endings starting with o/a in er/ir verbs

In er and ir verbs, the first person singular of the present indicative and the whole present subjunctive are the only forms whose endings start with o/a instead of e/i. These two different phonetic environments made Latin forms evolve differently in many verbs, leading to irregularities.

G-verbs

Before o (in the first person singular of the indicative present tense) and a (that is, in all persons of the present subjunctive), the so-called G-verbs (sometimes "Go-Yo verbs" or "Yo-Go" verbs or "Go" verbs) add a medial -g- after l and n (also after s in asir), add -ig- when the root ends in a vowel, or substitute -c- for -g-. This change overrides diphthongization (tener, venir) but combines with vowel-raising (decir). Many of these verbs are also irregular in other ways. For example:

ZC-verbs

This group of verbs—which originated in the Latin inchoative verbs but now includes other verbs as well—substitute -zc- for stem-final -c- before o and a. The group includes nearly all verbs ending in -acer (except ' and derived verbs), -ecer (except ' and '), -ocer (except ' and derived verbs), and -ucir. For example: ' may alternatively be conjugated with -zc- ('), -g- (G-Verb) (') or a compromise -zg- (').

Irregular forms in the future, conditional and imperative

Some -er and -ir verbs (most G-verbs plus ', ', ' and ') also change their stem in the future and conditional tenses. This involves syncope: Many of these verbs also have shortened tú imperative forms (apocope): tener → ', contener → ', poner → ', disponer → ', venir → ', salir → ', hacer → ', decir → '. However, all verbs derived from decir are regular in this form: bendice, maldice, desdícete, predice, contradice.

Anomalous stems in the preterite and derived tenses

Some verbs (including most G-verbs and most verbs ending in -ducir) have a somewhat different stem in the preterite. These stems are very old and often are found in Latin as well. The same irregular stem is also found in the imperfect subjunctive (both in -ra and -se forms) and the future subjunctive. These stems are anomalous also because: Examples: The verb in modern Spanish has a regular -er verb preterite (yo vi, tú viste, él vio – note the lack of written accent on monosyllables), but in archaic texts the irregular preterite forms yo vide, él vido are sometimes seen.

Irregular past participles

A number of verbs have irregular past participles, sometimes called "strong" because the change is in the root, rather than an ending. This includes verbs which are irregular in many other ways, as ' and ', but for some other verbs this is their only irregularity (such as ', '), while some very irregular verbs (such as ' and ') have regular past participles. Examples: Most of these verbs have derivatives with the same irregularity. For example, alongside volver → vuelto and poner → puesto, there are ' → ' and ' → '; alongside ' → ' there is ' → ' (but note ' → ', ' → ' are regular, though they also have the adjectival forms ' and '). Similarly ', ', ', ', etc. Solver is obsolete, but its derivatives ' and ' (', ') are in common use. The participle of ' is ' in some regions, but in others. There are three verbs that have both a regular and an irregular past participle. Both forms may be used when conjugating the compound tenses and the passive voice with the auxiliary verbs ' and ', but the irregular form is generally the only one used as an adjective: A number of other "strong" past participles, such as ', ', , and a number of others, are obsolete for general use, but are occasionally used in Spain (and to a much lesser extent in Spanish America) among educated, style-conscious writers, or in linguistic archaisms such as proverbs (refranes).

Others

The verbs ' (to be) and ' (to go) both exhibit irregularities in the present, imperfect and preterite forms (note that these two verbs have the same preterite fui). Together with ' (to see) and ' (to foresee), they are the only four verbs with irregular imperfect indicative. Their tú imperative forms are sé, ve (for both ir and ver, although is more common than ver in commands) and prevé. Their vos imperative forms are sé, andá (the verb replaces ir), ve and prevé. Remember that whenever the preterite is irregular, the imperfect subjunctive (-ra and -se forms) and the dated future subjunctive (-re) share the same irregularity; indeed, these tenses may always be correctly formed by substituting the appropriate endings for the -ron ending of the third person plural preterite: fueron > fuera/fuese...; fuere.... The verbs ' (to give) and ' (to be) both exhibit irregularities in the present indicative and present subjunctive because their stems cannot be stressed (in dar the stem is just d-, in estar it was originally st-). The form dé is so written to distinguish it from the preposition . Both verbs are also irregular in the preterite and derived tenses: dar follows the pattern of regular -er/-ir verbs, while estar has an anomalous preterite stem and follows the corresponding common pattern: