'à' ou 'de' , quand?

[Robert]

Dans ce qui suit, peut-on remplacer les prépositions 'à' et 'de' l'un par l'autre? Si non, pourquoi?

Ce livre est difficile à lire.
Il est difficile de lire ce livre.

Merci beaucoup.
Robert

[George Hunt]

I think you would not say "Ce livre est difficile de lire." but you can say "Il est difficile de lire ce livre."

Again you can say "Ce livre est difficile à lire." and I think you would not say say "Il est difficile à lire ce livre."

If I am correct.

[Robert]

Thank you so much.
But they are **prepositions** aren't they, same word in the title of your blog.

[Jacques Fabre]

Bravo pour votre réponse ! 

Avant de la lire, j'avais (en vain) compulsé mon "Grevisse" pour trouver la clé du mystère... Votre explication est parfaitement claire et convaincante.

[susan klee]

"La clé du mystère" ??? Pourquoi pas la clé au mystère ??

I've learned (so to speak) several languages, and I teach English to people from other language families, and I tell you, prepositions are murder  ;-))

Thank you, however, Madame le Professeur.  (And then there are the articles . . . )

[Jacques Fabre]

Susan,

In the expression "la clé du mystère", "mystère" is in the equilavent of a genitive case, as complement to the noun "clé". Genitives (namely possessive case : ~ 's) are normally introduced with preposition "de" (and its various forms : du - for "de le", masc. sing ; des - for "de les", masc./fem. plural).

Using "à" - "au" (contraction of "à le"), "aux" (contr. of "à les") to express a relation of possession belongs either to loose register of language (Ex. La voiture à mon père, inst. of "la voiture de mon père"), or to ancient usage, or to frozen expressions ("La bête à bon Dieu" - God's bug = ladybird ; "L'herbe à Nicot", Nicot's herb = tobacco, which is said to have been introduced into Europe by Jean Nicot, a French diplomat !).

"à" would be used in genitive case as the translation of  the English "to". (Ex. "Le monument aux Morts" -  monument to the Dead = war memorial), or "with"(namely in culinary language ; "Poulet à la crème" - chicken with cream sauce).

[Robert]

Can 'la clé au mystère' be not a possessive relationship at all? - an object not possessed, but somehow fitting, like a person being friend to another.