What is the difference between deber and deber de in Spanish?

Deber is used to express obligation or strong advice ("should/must"), while deber de suggests probability or assumption ("must be" in the sense of a guess). Example: Debes estudiar más. (“You should study more.”) vs. Debe de ser muy tarde. (“It must be very late.”)

Is deber de always necessary when expressing probability?

Not always. In modern usage, many native speakers drop the "de" and simply say debe ser even when expressing probability, although deber de is more traditional.

How do I conjugate deber in different tenses?

Deber is a regular -er verb. For example: Present : debo, debes, debe... Preterite : debí, debiste, debió... Conditional (for soft advice): debería, deberías...

Can deber be used in the conditional to make suggestions softer?

Yes! The conditional form debería means “should” in a more polite or less forceful way. Example: Deberías descansar. (“You should rest.”)

Is there a difference between deber and tener que ?

Yes. Tener que expresses a clear necessity or external obligation (“have to”), while deber often implies a moral duty or suggestion (“should/must”).

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B1

Grammar Point	Pattern	Example
Should in Spanish	Deber + infinitive (unconjugated form of the main action)	Debes mirar antes de cruzar la calle.
Must in Spanish	Tener que + infinitive	Tengo que irme a dormir temprano hoy.
Passive voice in Spanish	Subject + ser + past participle of the main verb + (optional agent complement)	Las piezas son fabricadas por la máquina.
Quoting someone in Spanish	Subject + Introductory Verb + “Quoted Speech”	Ella me preguntó: “¿Tienes prisa?”
If in Spanish	Si + present indicative (subordinate clause) / present indicative or imperative (main clause)	Si llueve, hay más accidentes de coche.
Instead in Spanish	En vez de + noun / infinitive verb	En vez de té, prefiero tomar café.
In order to in Spanish	Para + infinitive	Voy a clase de música para aprender a tocar el piano.
As soon as in Spanish	En cuanto + verb	En cuanto gane la lotería, me compro un coche.
Even though in Spanish	Aunque + verb	Aunque llueva, iremos al cine.
After all in Spanish	Después de todo + clause (assertion or conclusion)	Después de todo, ha sido más fácil de lo que parecía.
For now in Spanish	Por ahora, por el momento, de momento	Por ahora, no necesitamos más ayuda.
Although in Spanish	Aunque + main clause / Main clause + aunque	Aunque no me apetece, como sano.