Orthographical Changes
Due to the rules of Spanish orthography, some predictable changes are needed to keep the same consonant sound before a/o and e/i. This isn't usually considered as an irregularity. The following examples use the first person plural of the present subjunctive:
- c/qu: tocar > toquemos, delinquir > delincamos.
- z/c: gozar > gocemos, vencer > venzamos.
- g/j: proteger > protejamos. But in verbs ending in -jar, the j is kept before e: mojar > mojemos.
- g/gu: negar > neguemos.
- gu/gü: averiguar > averigüemos.
Other predictable changes involve stress marks, i/y alternances and i-dropping, some of them are sometimes considered as irregularities. The examples are several forms of otherwise regular preterites:
- Stress mark on stressed i after a/e/o: caer > caímos, leer > leímos, oír > oímos.
- Stress mark not used in monosyllabic forms: fiar > fie (rather than' *fié), fio; ver > vi, vio.
- Unstressed i is written y between vowels: caer > cayó, cayeron.
- Unstressed i is dropped between ll/ñ and a vowel: bullir > bulló (not *bullió"), tañer > tañó.
Read more about this topic: Spanish Irregular Verbs