'Avoir besoin de' or 'devoir'?

In French, you will probably have come across two different ways to express necessity: devoir and avoir besoin de.

While these two terms are often interchangeable, there are some instances where only one can be used, so it's important to know the difference.

Avoir besoin de roughly translates as 'to have need of'. We use the expression avoir besoin dewhen we want to express need of a particular noun, for instance:

J'ai besoin de votre aide pour ça. - I need your help with this.

Elle a besoin de nouvelles piles pour sa lampe de poche. - She needs new batteries for her torch.
We can also use besoin to express necessity for a verb, as in the following examples:

Je vais avoir besoin de manger plus de fruits. - I will need to eat more fruit.

Tu as besoin de faire tes devoirs avant le dîner. - You should do your homework before dinner.
When used with a verb, avoir besoin de generally denotes a personal desire for something to be done, rather than an obligation. The implication is that the action would be useful or desirable, rather than compulsory.

Remember that the verb here is avoir, and so it must conjugated with the correct person and tense.

Devoir translates as 'to owe' or 'ought'. When we use it with a verb, it means 'ought to', or 'must', as in the following examples:

Vous devez être plus attentifs en classe. - You ought to pay more attention in class.

Nous devons réfléchir à l’endroit où nous voulons vivre à l’avenir. - We need to think about where we want to live in the future.

As you can see, when followed by a verb, devoir is very similar to avoir besoin de, but with a stronger sense of the action being obligatory.

Note that you cannot use devoir with a noun to denote necessity, as you can do with avoir besoin de. When devoir is followed by a noun, it means ‘to owe’:

Je te dois dix euros. - I owe you ten euros.

Tu me dois un nouveau vélo si tu endommages celui-ci. - You owe me a new bike if you damage this one.

Both devoir and avoir are irregular verbs. For more on how to conjugate irregular verbs, see Lesson 11 of The Complete French Course.