preterite verbs - third person e->i and o->u
Remember -- these conjugate with normal -ir preterite verb endings (-í, -iste, -ió, -imos,-steis, and -ieron), they just have this little change in the él/ella/Ud. form and the ellos/ellas/Uds. form.
Oh, and reír and sonreír have a couple extra accents.