Unlike –ar and –er stem-changing verbs, –ir verbs show a one-letter stem change (the first letter if the verb has a two-letter change in the present tense) in the third persons of the preterite, as they do in the present participle.
| Pedir (i) | Dormir (ue) | |
|---|---|---|
| yo | pedí | dormí |
| Tú | pediste | dormiste |
| él, Ella, Ud. | pidió | durmió |
| Nosotros | pedimos | dormimos |
| Vosotros | pedisteis | dormisteis |
| ellos, ellas, Uds. | pi>dieron | durmieron |
Present participles: pidiendo, durmiendo
Other verbs have orthographic changes that appear in the first-person singular of the present tense. All verbs ending in -car, -gar and –zar undergo this change.
| sacar | saqué |
| llegar | llegué |
| comenzar | comencé |
Once you recognize the infinitive and the tense, none of the above changes should present comprehension difficulties.