The Spanish constructions haber + de, tener + que, hay + que, and deber + infinitive are all used to express that something demands attention or action: an obligation, requirement, or necessity. Let's review some examples to learn the subtle differences between them and how to properly use them.

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The construction haber + de + infinitive is used to express a mild sense of obligation or necessity (in some contexts it could be just a possibility). Its use is, therefore, preferred when you want to give an instruction in a very polite way, making it sound more like a suggestion than an order. For example, in one of our new videos Raquel uses haber + de + infinitive repeatedly to share some entrepreneurial tips:
 

En primer lugar, hemos de definir nuestra estrategia.

In the first place, we have to define our strategy.

Caption 5, Raquel y Marisa - Español Para Negocios - Introducción

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Básicamente, en esta parte, hemos de definir

Basically, in this part, we have to define

qué vamos a publicar en cada red social.

what we are going to publish on each social network.

Captions 8-9, Raquel y Marisa - Español Para Negocios - Introducción

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Additionally, by using the first person plural, hemos, Raquel gives a subtle aura of consensus to her advice, which stresses the idea that even when she is using an imperative expression, she is not giving an order but rather sharing advice. If she were to use, for example, the second person, the expression would have a bit more pressing or demanding tone:
 

Lo primero que has de hacer al reservar en un restaurante es...

The first thing that you have to do upon reserving at a restaurant is...

Caption 3, Raquel - Reserva de Restaurante

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Here's another example with a twist, using negation:
 

Y no ha de faltarle nunca

And you should never lack

un sanconcho de ñato pa' rematar.

a snub-nosed fish stew to add the finishing touch.

Caption 14, Mary Grueso Romero - Platonera en la plaza del mercado

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You could also add the word uno (one), to talk impersonally. For example:

Uno ha de hacer aquello que desea.
One must do whatever one wants.

Incidentally, there is one Spanish imperative construction that only uses the impersonal form to express needs, obligations, or requirements in a more generalized way: hay + que infinitive.
 

¡Eres una víbora a la que hay que quitarle la ponzoña!

You're a snake from which it's necessary to remove the venom!

Caption 27, El Ausente - Acto 3

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Or for everyday tasks at hand:
 

Pues hay que diseñar unos 'flyers'.

So, we have to design some flyers.

Caption 58, Arturo Vega - Entrevista

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On the other hand, to convey a higher grade or urgency, necessity, or imperativeness, the expressions tener + que + infinitive and deber + infinitive* come in handy. The difference between deber and tener is subtle: the use of deber confers a sense of duty or moral imperativeness to the expression, while tener is better suited to talk about more practical matters.

You can also use these constructions in an impersonal way, adding the word uno (one):
 

Ya, cuando a uno le toca ser papá, pues,

Then, when it's time for one to be a dad, well,

uno tiene que reflexionar sobre eso.

one has to reflect on that.

Caption 8, La Sub30 - Familias

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[If you want to use the verb deber you would say: uno debe reflexionar (one must reflect)]

Or you can use the first person for more particular and pressing needs:
 

Ahora tenemos que hablar de precio.

Now we have to talk about price.

Caption 74, 75 minutos - Gangas para ricos

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[If you were to use the verb deber: debemos hablar de precio (we must talk about price)]

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*By the way, the use of deber + de + infinitive in imperative statements (such as debes de comer, meaning "you must eat") is common but grammatically incorrect. The use of deber + de + infinitive is correct when used to express probability.