"Deber" id used when something must be some way.
"Deber de" is used when one makes a guess about something one expects to be some way.
Debo terminar mi trabajo esta tarde.
I must finish my work this afternoon.
Había demasiados errores en mi trabajo. Debo de haberme distraído.
There were too many errors in my work. I must have been distracted.
Debes ir a ver a tu madre al hospital.
You must go and see your mother in the hospital.
Encontré a la señora inconsciente en el suelo. Debe de haberse caído de la escalera.
I found the lady unconscious on the floor. She must have fallen from the stairs.
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