The verb falloir is only ever used in the third person singular impersonal expression Il faut.
You will NEVER see je faux or nous fallons for example!
Depending on context, it can express either necessity or obligation.
When you use il faut, you are talking about a general rule, or a general statement that applies to people.
Look at these examples:
Il ne faut pas marcher sur la pelouse.You mustn't walk on the grass.
Il faut un accent sur le premier 'e' de "école"You need an accent on the first 'e' of "école"
Il faut une gomme pour effacer des erreurs.We need a rubber / eraser to erase mistakes.
Indeed, it can mean you must (in general), we (people) must or one must OR you need (in general), we (people) need or one needs.
Il faut can be followed by a verb in the infinitive or a noun:
- Il faut + infinitive = to need to [do something] OR must / to have to [do something]
- Il faut + noun = to need [something]
Il faut faire ses devoirs.One must do one's homework.
Note that when using the general il faut, the possessive pronoun will be son, sa, ses as it is quite similar to saying One must...
You can also use il faut in a personal way, to state things that a specific person has to do or needs (to do), but the structure changes slightly.
Look at these examples:
Il me faut un crayon.I need a pencil.
Il lui faut de l'aide.He needs some help.
She needs some help.
Il vous faut trois œufs pour cette recette.You need three eggs for that recipe.
Il leur faut quitter cet endroit.They must leave this place.
They need to leave this place.
Note that to use il faut for specific people, you need to use an indirect object pronoun (me, te, lui, nous, vous, leur), as such:
- Il [pronoun] faut + noun = to need [something]
(Literally, it needs to me/ to him / to them OR it must to me/ to him/ to them)
- Il [pronoun] faut + infinitive = to need to [do something] OR must / to have to [do something]
Note that the latter structure sounds very formal in French! In everyday language, you would usually use one of the alternatives listed below.
Il faut ranger ta chambre.You must tidy your room. Il faut ranger sa chambre.You must tidy your room.
-> In the first case, you refers to a specific person, therefore you need to use the possessive ta, whereas in the second case, you refers to people in general, and you'll use the impersonal possessive sa.
And see also other ways to express necessity or obligation:
Avoir besoin de = To need (French Expressions with avoir)
Conjugate devoir in the present tense in French (Le Présent)
and the more advanced usage of il faut:
Using Il faut que with the subjunctive mood (Le Subjonctif) in French
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- Slang/Expression/Highly Idiomatic