Take a look at the conjugation of deber (must) in El Presente:
| yo |
debo |
| tú |
debes |
él / ella / Ud.
|
debe |
nosotros / nosotras
|
debemos |
vosotros / vosotras
|
debéis |
ellos / ellas / Uds.
|
deben |
The endings are the same as regular -er verbs: Conjugate regular -er verbs in El Presente (present tense).
Look at and listen to these examples:
Debo trabajar ocho horas al día.I must work eight hours per day.
Debes usar crema solar en la playa.You must use sunscreen at the beach.
Pablo debe estudiar todas las tablas de memoria.Pablo must study all the multiplication tables.
Nosotros debemos venir al colegio a las ocho.We must come to school at eight.
Vosotras debéis respetar a los profesores.You must respect the teachers.
Ustedes deben llegar puntuales.You must arrive on time.
Note that in the examples above, the modal verb deber is followed by an infinitive.
See also Using deber, poder, querer + infinitive to express must, can, to want to.
Ustedes deben llegar puntuales.You must arrive on time. Vosotras debéis respetar a los profesores.You must respect the teachers. Nosotros debemos venir al colegio a las ocho.We must come to school at eight. Debes usar crema solar en la playa.You must use sunscreen at the beach. Debo trabajar ocho horas al día.I must work eight hours per day. Pablo debe estudiar todas las tablas de memoria.Pablo must study all the multiplication tables. 
