- Yo duermo 8 horas.
- Yo tengo un perro.

Remember that stem-changing verbs there is a subtype that is the verbs o-ue.
These change, apart from the endings of the conjugations, the roots of the verbs themselves. As "dormir" (sleep) change the "o" to "ue".
Remember that yo-go verbs are verbs whose endings are "-go" always with the tonic pronoun "yo" (I).
In this case: