Of the 3 categories of Spanish infinitives, IR infinitives are most likely to have irregular spellings--across all verb tenses. Many of the irregularities are predictable--based on the presence of irregulars in the present tense.
For example, stem changing IR verbs in the present tense are guaranteed to have irregular spellings in the preterite tense--but only in the él/ella/Usted and ellos/ellas/Ustedes conjugations.
Likewise, the present participles of IR verbs are certain to be irregular when the present tense conjugations are irregular. The table below represents this reality:
PRESENT TENSE CONJUGATIONS
| DORMIR | Duermo | Duermes | Duerme | Dormimos | Dormís | duermen |
| PEDIR | Pido | Pides | Pide | Pedimos | Pedís | piden |
| VESTIR | Visto | Vistes | Viste | Vestimos | Vestís | Visten |
| PRODUCIR | Produzco | Produces | Produce | Producimos | Producéis | Producen |
| IR | Voy | Vas | Va | Vamos | Vais | van |
PRETERITE CONJUGATIONS
| DORMIR | Dormí | Dormiste | dUrmió | Dormimos | Dormisteis | dUrmieron |
| PEDIR | Pedí | Pediste | pIdió | Pedimos | Pedsisteis | pIdieron |
| VESTIR | Vestí | Vestiste | vIstió | Vestimos | Vestisties | Vistieron |
| PRODUCIR | Produje | Produjiste | Produjo | Produjimos | produjisteis | produjeron |
| IR | fui | Fuiste | Fue | Fuimos | Fuisteis | fueron |
PRESENT PARTICIPLE
| DORMIR | durmiendo |
| PEDIR | pidiendo |
| VESTIR | vistiendo |
| VENIR | viniendo |
| IR | yendo |
Bottom line, the spelling of IR verbs more often "GO crazy/VAN loco"--across all tenses than AR and ER verbs. Of course, this might be expected since IR is the Spanish infinitive for TO GO. ¡OJO!
For more examples, check out the irregular spellings across tenses for the following additional samples of "crazy/loco" IR verbs:
MORIR to die
MEDIR to measure
DESPEDIRSE to bid farewell
REPETIR to repeat
VESTIR to dress
FREIR to fry
REIR to laugh
SEGUIR to follow/continue
CONSEGUIR to get
COMPETIR to compete
DERRETIR to melt
