"il faut" is not "he must"?
I translated "Il faut avoir un gros livre." as "he must have a big book", but that's considered wrong.
Apparently "il faut" does not apply to a person, it's just a general "it must be so".
So "he must have .." is "il doit avoir ...".
In practice, is "falloir" being used with anything but "il"?
That's right! In fact, there isn't even a way to conjugate "falloir" with any person besides "il." Think of it like the "il" in "il y a" or "il fait beau"--it is completely general, and you might translate it as "it" or "there." If you need to designate a specific person, you can say "il faut que je/tu/elle..." and then put the verb in the subjunctive. So to say "he must have a big book," you could say "il faut qu'il ait un gros livre." "Il doit avoir" works just as well, though. (I learned that "il faut" and "devoir" are basically synonyms, but "il faut" is stronger--maybe someone else can expand on that?)
anomalocaris: I'm not sure that "falloir" is stronger than "devoir". As you said, the difference is that "falloir" does not apply to a speciefied person, while "devoir" usualy do. In my mind, "Il faut" and "On doit" is exactly the same, but "Il faut" and "Il doit" is not, because when you say "Il doit", you are thinking of someone in particular. For something a little bit less strong, you can use the conditionnal of falloir : "Il faudrait". "Il faudrait" is less strong and can mean also that you know it should be done, but you don't really want to.
@anomalocaris : Personnaly, i'd rather use "il faut", for three reasons:
- "on doit" can also mean "nous devons" in common speach
- "on doit" feels a little bit more imperative. "on doit" makes me think of an command, while "il faut" feels more like saying a fact. Maybe because "on doit" can mean "nous devons" while "il faut" is not specified to any person. But it is really subtile and really both can be used for either case. It's like saying "on doit faire la vaisselle" (we must do the dishes) and "il faut faire la vaisselle" (the dishes must be done). Most of the times, in both case, it is implicit who must do the dishes, so there is no real difference.
- "il faut" just seems more natural, but I suppose it might be cultural.
But it is more a preference, really not a rule. I can't really think of a situation that one would be weirder than the other.