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The verbs ser (to be) and ir (to go) both exhibit irregularities in the present, imperfect and preterite forms. Together with ver (to see) and prever (to foresee), they are the only four verbs with irregular imperfect indicative.
| Present indicative tense | Imperfect indicative | Preterite | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| ser | ir | ver | prever | ser | ir | ver | prever | ser/ir | ver | prever | ||||
| yo | soy | voy | veo | preveo | era | iba | veía | preveía | fui | vi | preví | |||
| tú | eres | vas | ves | prevés | eras | ibas | veías | preveías | fuiste | viste | previste | |||
| vos | sos | |||||||||||||
| él, ella | es | va | ve | prevé | era | iba | veía | preveía | fue | vio | previó | |||
| nosotros/as | somos | vamos | vemos | prevemos | éramos | íbamos | veíamos | preveíamos | fuimos | vimos | previmos | |||
| vosotros/as | sois | vais | veis | prevéis | erais | ibais | veíais | preveíais | fuisteis | visteis | previsteis | |||
| ellos/as | son | van | ven | prevén | eran | iban | veían | preveían | fueron | vieron | previeron | |||
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 dar (to give) and estar (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 de. 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:
| Present indicative | Present subjunctive | Preterite | |||||||
|---|---|---|---|---|---|---|---|---|---|
| dar | estar | dar | estar | dar | estar | ||||
| yo | doy | estoy | dé | esté | di | estuve | |||
| tú, vos | das | estás | des | estés | diste | estuviste | |||
| él, ella | da | está | dé | esté | dio | estuvo | |||
| nosotros/as | damos | estamos | demos | estemos | dimos | estuvimos | |||
| vosotros/as | dais | estáis | deis | estéis | disteis | estuvisteis | |||
| ellos/as | dan | están | den | estén | dieron | estuvieron | |||
Read more about this topic: Spanish Irregular Verbs