"He must be tired."
Translation:Él debe de estar cansado.
OK, this is a tricky one, such a minor nuance! But I'll try to explain.
Deber + infinitive = obligation, sthg that must/should be done
Él debe hacer su tarea = He must do his homework
No debemos cruzar el camino aquí = We must not cross the road here
Deber de +infinitive = possibility/probability that something must have happened or, (as in this case with estar + adjective) that a person must be in a certain state.
Tu debes de estar cansado... después de caminar todo el día = You must be tired... after walking all day.
Está oscuro afuera. Debe de ser muy tarde = It's dark outside. It must be very late.
As you can see, these last two examples have nothing to do with 'obligation' as the word 'deber' usually implies.
Also note: as this usage of deber usually goes onto express an opinion that may or may not be true, it might seem to hint towards the use of the subjunctive. But this is not the case, as the speaker is sure of their statement in their own mind and it would need a 'that' statement - something like 'creo que...' (I believe that...) - to trigger the subjunctive here, which would then make the 'deber' redundant anyway.
Thank you very much! We need more people like you on the platform
Él debe estar cansado... I am a native Spanish speaker, and this sentence is correct !
I got this wrong too but "unfortunately" it is correct... ;-)
el verbo deber forma dos perífrasis verbales de infinitivo:
• deber, sin de, implica obligación o necesidad, que es el significado preferido, pero puede expresar suposición o creencia;
• deber de solo expresa suposición o creencia.