The Difference Between 'Devoir' and 'Falloir'

The French verbs devoir and falloir can be confusing because they both express obligation and necessity but in different ways. In addition, each verb has a different meaning when followed by a noun. Both devoir and falloir are extremely irregular verbs, and both are very common, perhaps the third person singular of falloir—il faut—most of all. The conjugation of both should be memorized because the French speaker is sure to need them on a daily basis.

Devoir

When followed by an infinitive, devoir expresses obligation, probability or supposition.

   Je dois partir.
   I have to; I must; I'm supposed to leave

   Je devais étudier.
   I had to; I was supposed to study.

   Je devrai travailler.
   I will have to work.

   Je devrais lire.
   I would have to; I should read.

   J'ai dû manger.
   I had to eat; I must have eaten.

   J'aurais dû manger.
   I should have eaten.

When followed by a noun, devoir means "to owe."

   Je dois 5 dollars.
   I owe 5 dollars.

   Je ne lui devais rien.
   I didn't owe him anything.

Falloir

Falloir is stronger and somewhat more formal than devoir; it expresses necessity. Falloir can be used with an infinitive or the subjunctive. Because it's an impersonal verb, falloir does not conjugate for different subjects. So in order to specify the person who needs to do something, you can either use the subjunctive or an indirect object pronoun with the infinitive.
   Il faut travailler
   It is necessary to work; we need to work.

   Il me faut travailler; Il faut que je travaille.
   I need to work.

   Il ne faut pas manger.
   We mustn't eat.

   Il nous fallait manger.
   We had to eat.

   Il ne nous faut pas manger; Il ne faut pas que nous mangions.
   We don't need to eat, We mustn't eat.

When used with a noun, falloir means "to need."
   Qu'est-ce qu'il te faut?
   What do you need?

   Il me faut un stylo.
   I need a pen.