Haber de + infinitive to express formal obligation or future action (perífrasis verbal)

Although it is more common to find it in literature and in formal content, in Spanish we can use the auxiliary verb haber + de + infinitive. This verbal structure is used in two different ways:

Haber de + infinitive: obligation, convenience, necessity

It expresses an obligation although not a strong one, or the convenience or necessity of doing something. 

Have a look and listen to these examples:

He de coger el tren de las diez para llegar a tiempo a mi destino.I must take the ten o'clock train to arrive at my destination on time.

Han de respetar todas las señales de tráfico.You must respect all traffic signs.

Hemos de admitir que es más duro de lo que pensábamos.We must admit that it is harder than we thought.

Has de reconocer que los estudiantes merecen un descanso.You must admit that the students deserve a break.

Its more colloquial and informal equivalent is:

tener que/ deber/necesitar + infinitive

Tengo que/Debo/Necesito coger el tren de las diez para llegar a tiempo a mi destino.

Haber de + infinitive: future action. 

Have a look and listen to these examples:

Esta guerra habrá de acabar con la esperanza de paz de la gente.This war will put an end to people's hope for peace.

No te preocupes por lo que nunca ha de pasar.Don't worry about what will never happen.

Its more colloquial and informal equivalent is:

the main verb in the future tense

Esta guerra acabará con la esperanza de paz de la gente.

 

¿Por qué habría de + infinitive? Why should/would...?

There is a slightly different connotation when we use "habría de" (i.e. haber in the conditional tense) in a question using ¿Por qué...? 
This is generally used indicating "why should/would something have a certain effect", normally within a conversation where there is an element of doubt about something. 

For example:

¿Por qué habría de afectarme a mí esa decisión?Why would / should that decision affect me?

¿Por qué habría de tener un impacto negativo? Justo al contrario.Why would it / should it have a negative impact? Quite the opposite.

¿Por qué habría yo de molestarme? Tranquilo, lo entiendo.Why should I / would I get offended? Don't worry, I understand.

 

Remember:

Haber (conjugated) + de + infinitive

 

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Examples and resources

Has de reconocer que los estudiantes merecen un descanso.You must admit that the students deserve a break.
Esta guerra habrá de acabar con la esperanza de paz de la gente.This war will put an end to people's hope for peace.
No te preocupes por lo que nunca ha de pasar.Don't worry about what will never happen.
He de coger el tren de las diez para llegar a tiempo a mi destino.I must take the ten o'clock train to arrive at my destination on time.
Hemos de admitir que es más duro de lo que pensábamos.We must admit that it is harder than we thought.
¿Por qué habría de tener un impacto negativo? Justo al contrario.Why would it / should it have a negative impact? Quite the opposite.
¿Por qué habría de afectarme a mí esa decisión?Why would / should that decision affect me?
¿Por qué habría yo de molestarme? Tranquilo, lo entiendo.Why should I / would I get offended? Don't worry, I understand.
Han de respetar todas las señales de tráfico.You must respect all traffic signs.
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