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.
Some verbs, in their various forms, can exhibit both kinds of changes (e.g. sentir, siente, sintió (e-ie-i); dormir, duerme, durmió) (o-ue-u).
Common non-diphthongizing verbs include acercar(se), beber, comer, comprar, conocer, correr, creer, deber, dejar, entrar, esperar, lamentar, llegar, llevar, meter, parecer, poner, prometer, quedar, regresar, responder, suceder, temer, and tomar.
The stressed vowel is marked bold in the examples: cambiar > cambio, but enviar > 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: anunciar, averiguar, bailar, causar, peinar and adeudar for diphthong-keeping verbs and enviar, actuar, aislar, aunar, descafeinar and rehusar for diphthong-breaking ones.
Diphthong-breaking verbs include ahincar, aislar, aunar, aullar, maullar, aupar, aliar, vaciar, contrariar, evaluar, habituar, reunir.
The verbs criar, fiar, guiar, liar and piar are also diphthong-breaking (crío, guíe), 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 licuar and adecuar both options are valid: adecuo or adecúo.
This also applies to the forms of oír and desoír that do not undergo the -ig- change: oyes, oye, oyen… Some regular forms of fluir, fruir and huir 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).
Some -er and -ir verbs (most G-verbs plus haber, saber, poder and querer) 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 → ten, contener → contén, poner → pon, disponer → dispón, venir → ven, salir → sal, hacer → haz, decir → di.
These stems are anomalous also because: Examples: The verb ver 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, etc.
Both forms may be used when conjugating the compound tenses and the passive voice with the auxiliary verbs haber and ser, but the irregular form is generally the only one used as an adjective: A number of other "strong" past participles, such as pinto, ducho, electo, 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).
Their vos imperative forms are sé, andá (the verb andar replaces ir), ve and prevé.