These verbs are known as o to ue stem-changing verbs.
No recuerdo. - I don't remember. No lo recuerdo. - I don't remember it. No recuerdo su nombre. - I don't remember his/her name. El niño recuerda los números. - The young boy remembers the numbers. Ellas recuerdan a su hermana. - They remember their sister. Él no recuerda el incidente. - He does not remember the incident. Él recuerda los viejos tiempos. - He remembers the old times. Tú recuerdas a tu amigo. - You remember your friend. Nosotros recordamos las buenas memorias. - We remember the good memories. Mi abuela no recuerda nada. - My grandmother doesn't remember anything.