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, 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
Conjugations of devoir
All about devoir - meaning changes in different tenses and moods
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, One needs to work.
Il me faut travailler, Il faut que je travaille
I need to work.
Il ne faut pas manger
One must not 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.
Conjugations of falloir
Also see All about falloir
|Type of verb||personal||impersonal|
|Meaning when followed by...|
|infinitive||must, to have to||to be necessary/need to|
|subjunctive||- - -||to be necessary/need to|
|noun||to owe||to need|
See the French grammar glossary if you don't understand any of these terms.