Making assumptions with "deber de"
When you want to deduce something based on clues—like saying "it must be..." or "he must have..." in English—use the verb deber followed by the preposition de and an infinitive.
You already know that deber on its own means "should" or "must" when talking about obligations (for example, debo estudiar = I must study). However, adding the preposition de shifts the meaning entirely from obligation to probability or supposition.
| It must be water, because there is a puddle on the ground. | Debe de ser agua, porque hay un charco en el suelo. |
| The girl has tears in her eyes, she must be sad. | La niña tiene lágrimas en los ojos, debe de estar triste. |
Deducing from sensory clues
This structure is perfectly suited for drawing logical inferences from things you see, hear, or smell in your environment. You will frequently see deber de paired with the verbs ser (to be), estar (to be), and haber (to be / to exist).
| There must be a fire, because I see a lot of smoke. | Debe de haber un incendio, porque veo mucho humo. |
| I hear a very loud siren, there must be a crash. | Escucho una sirena muy fuerte, debe de haber un choque. |
| There must be bread because there is a very delicious smell. | Debe de haber pan porque hay un olor muy rico. |
| It must be the mail carrier because the dog is barking. | Debe de ser el cartero porque el perro ladra. |
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