devoir or falloir?

[Billy Bosworth]

Is there any difference in the meaning of these 2 sentences?

1. Il me faut aller à Londres. (I have to go to London).
2. Je dois aller à Londres. (as above)

Can both forms be used interchangeably?

[George Hunt]

To my mind they are more or less interchangeable.

That is not to say that they don't sound different. But I personally think I would use whichever form seemed better at the time.

I would be interested to learn if there were circumstances when it would make a noticeable difference depending on which form might be used .

[Sandra]

You are right George, they are perfectly interchangeable.

The choice of one or the other will not depend of the context but of you, your "level" of language, how you want to appear and who you are speaking to.

As you may know in French we have different "levels" of language from the most sophisticated (in general all the ancient forms + literature) to the worst spoken French called "Argo"

In between you have many more subtle "levels", their use doesn't respond to any rule or logic, I think only experience (and a very good knowledge of French) can help you choose how you want to express yourself.

In this case : "Il me faut" is the sophisticated form of "je dois". This form is not used anymore in current or professional conversation or writing, it sounds  archaic, a little bit "chivalrous" I would say....

But your sentence is not entirely  "high level" because "aller à londres" is "standard level", the sophisticated form is "me rendre à Londres" (or "rejoindre Londres" or  "rallier Londres")

Il me faut me rendre à Londres (very high level)

Il me faut aller à Londres (half high level)

Je dois me rendre à Londres (half high level)

Je dois aller à Londres (standard)

Faut qu' j'aille à Londres (Argo)

The meaning is exactly the same.... welcome to the wonderful world of French nuances  :)

NB: This is my opinion as a native speaker in 2015. The general "level" of French is decreasing very fast, "Very high level" was still in use 50 years ago. That's why you may encounter it a lot in literature or any other text you may study. But the sad truth is that today people will make strange faces if you use "Il me faut me rendre à Londres"  because you will sound like  a book/film protagonist.

[George Hunt]

Thanks Sandra . That was an interesting observation. It is really the first time I have learned about these "levels" even though I am familiar with French "argot" .

Of course it does not come as a surprise that there should be these degrees of sophistication since the same or similar phenomenon exists in English.

[Billy Bosworth]

Sandra, Many thanks for such an enlightening (half high level?) and comprehensive answer. Absolutely amazin' (Argo?). 

[Sandra]

Hahaha my level of English is not good enough to answer you !

By the way I made a big mistake, "Argot" has a silent T in the end (George is right) .. I never knew why ...

All languages  are evolving and can be used at different "levels" but I think "Argot" got something unique that makes it very difficult to understand for non native speakers. You see in Argot, we don't only contract words or use pretty bad grammar but we also replaces words by others with a total different meaning...

For exemple a car , "voiture" in argot is "une caisse" (don't ask me why XD )

A woman, "femme" is "une meuf" (this one is "femme" pronounced backward)

A man, "un homme" is  a "mec" or "keum" ("mec" pronounced backward)

This is a worst form than "gars" = "dude"

Police is called "poulets" (chickens) or "Keuf" ("fuck" pronounced backward)

To like something, "aimer" = "kiffer" (Frenchifying of the arab word "kiff")

We also Frenchify English words like "to book" = "booker" the meaning remains the same...

To eat , "manger" = "bouffer" (I don't know why)

And it goes on and on and on .....

[Crack1]

In this context, I thought it would be ask about the words ought and must.

For me, the English word must is stronger than ought.

I am not a native English speaker. Our friends whose mother tongue is English may disagree with me.

I must go to the gym tomorrow.

I ought to go to the gym tomorrow.

How do you translate the above 2 sentences?

You may want to know the context.

Let us assume that a friend invited to have lunch at a restaurant.

[George Hunt]

You can use the conditional tense to soften the sense.

"je devrais" instead of "je dois" and also "il me faudrait " instead of "il me faut".

[Billy Bosworth]

It gets interesting when we consider the negatives of "have to" and "must" ...

I don't have to ... Il ne me faut pas ... (?)

but the negative of "must" can have 2 meanings ...

1. I must not (in the sense of forbidden) ... Je ne dois pas (?)
2. I don't need to (in the sense of not necessary) ... Je n'ai pas besoin de  (?)

I would appreciate any comments/corrections to the interpretations above. 

[Jean Dervin]

"Bouffer" is onomatopoeic (from non-classical Latin "buffare") and has the notion of gobbling something or blowing with swollen cheeks.  Hence the meanings (1) eat or soak up and (2) to be anti something (e.g. "il bouffe du communiste").

There's an excellent chapter on "Register" (what Sandra calls "levels") in Batchelor and Offord, "Using French", Cambridge University Press.  (I have the 3rd edition of 2000 .)