Conjecture and obligation in Spanish

The idea

Deber de plus an infinitive commonly expresses conjecture; tener que plus an infinitive expresses obligation or necessity. In actual usage, deber without de can also express conjecture.

Examples

• La luz está encendida; deben de estar en casa.The light is on; they must be home.
• La calle está mojada; debe de haber llovido.The street is wet; it must have rained.
• El paquete debe de pesar dos kilos.The parcel probably weighs two kilos.
• Tenemos que entregar el informe hoy.We have to hand in the report today.
• El personal tiene que llevar una identificación.Staff have to wear identification.
• Tengo que cerrar la ventana antes de salir.I have to close the window before leaving.

Watch out

Do not assume that deber de always expresses an obligation, or that every speaker keeps the traditional distinction.

Try it yourself

1. The floor is wet: express a conjecture. El suelo está mojado; ___ haber llovido. (debe de / tiene que)

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   debe de

   debe de: the wet floor suggests rain.

2. A rule requires you to wear a helmet: express obligation. Por norma, tú ___ llevar casco. (debes de / tienes que)

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   tienes que

   tienes que: the rule applies to you.

3. It is very late: express a conjecture. Es muy tarde; ___ de estar cerrado. (debe / tiene)

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   debe

   debe: the existing de completes the conjecture.

Use it today

Choose the requested meaning and the form that agrees with the subject.