Spanish Verb Conjugation
The majority of experiments on learning regular versus irregular forms in second language grammar come from English past tense. However, in English only regular past tense forms show affixation (e.g., walk -> walked); irregulars show a transformation of the stem itself (e.g., go -> went). This means that regular / irregular forms in English differ not only in regularity, but also in the need for an affix.
Experiments with languages like Spanish can avoid this confound. Spanish shows a rich inflectional morphology system, and both regular (no transformation to the stem) and irregular (some transformation) require affixes when conjugating the verb.
Types of Irregularity
In traditional dual-route models of morphological processing (e.g., Pinker, 2000), there is one significant dissociation in morphology, between regulars that can be (but may not be) composed from the stem and appropriate regular affix, and irregulars that are always retrieved by rote from declarative memory.
However, the types of irregularity in Spanish could complicate the picture of regular vs. irregular morphological inflection. For example, there are verbs that follow a "stem change" pattern (e.g., colgar "to hold"); the transformation of the stem is limited to certain inflectional forms (1st person singular cuelgo takes a transformation, while 1st person plural colgamos does not), and the pattern of transformation is shared across all stem-change verbs.
Also, there are "spelling change" verbs, which change orthographic form to preserve a sound in the verb stem. Verbs that end in -car, for example, show a hard 'c' sound in the infinitive (e.g., colocar, "to place"). When conjugated, the affix rules can lead to a vowel next to the 'c' that creates a soft 'c' sound (i.e., an 'e' or 'i'). In these cases, the spelling changes from 'c' to 'qu' to preserve the pronunciation (e.g. 1st person singular for present subjunctive coloque).