What's the difference between devoir and avoir à?

Question

I feel like sometimes I've seen these used in an interchangeable manner (devoir and avoir à). What I told my students was that "devoir" also means "must", so "avoir à" needs to be used in situations where "must" doesn't fit (i.e. "You don't have to go yet." "Tu n'às pas encore à partir."). Is this correct? In what other situations do you need to use one over the other?

Accepted Answer

Bonjour,
 
Chris is right about ‘devoir’ being used for general obligation. It is also the best option. Indeed, “you’ll not go wrong”. Although there is a slight difference.
‘Devoir’ can be used to express a necessity, an obligation, a strong recommendation or a general obligation:

Je dois dépoussiérer toute la maison tous les mois = I have to dust the whole house every month
Tu dois faire ce que je te dis = You must do what I tell you to
Vous devez faire plus d’efforts la prochaine fois = You must try harder next time

‘Avoir [qqch] à faire’ is mostly used in a conversation. It conveys a sense of obligation that is more or less imperative / absolute and « for concrete actions » as Chris mentioned :

J’ai des choses à faire = I have things to do
Nous avons des détails à régler = We have some things to finish off
Il avait un tas de lettres à écrire = He had a bunch of letters to write

You can also write it this way : avoir à + [infinitive verb] +[qqch] – but it is far less common and mostly found in written work of fiction etc:

J’ai à régler des détails = I have to tie up some loose ends
Elle avait à écrire une lettre importante = she had to write an important letter

I hope this is helpful.
Bonne journée !

Answer

If you stick with devoir, you'll not go wrong. There are some cases where you could use avoir à, but that doesn't work all the time and is also of a bit of a lower register, i.e., more colloquial, as I understand it.

"Je dois aller à l'ècole" is correct, but you CANNOT say "j'ai à aller à l'école." You CAN use it in the sense of "J'ai une lettre à écrire", "J'ai qqc à faire ce matin."

Thinking about it some, I think you could use avoir à for concrete actions. Going to school is a general obligation, where devoir is needed. But Céline's or Cecile's input would be greatly appreciated on this.