C'est/Il est + Adjective + à/de + Infinitive

C'est + adjective + à + infinitive is used when the
idea has already been mentioned; while il est + adjective + de + infinitive
is used when the idea has not yet been mentioned. Also, the c'est construction
is used when you do not use a direct object after the infinitive of the transitive
verb, and the il est construction is used when you do.

Est-ce qu'on peut apprendre le chinois en un an ? Can you
learn Chinese in one year?

Non, c'est impossible à apprendre en un an ! No, it's impossible
to learn in one year! (The idea, Chinese, has already been mentioned, and there
is no direct object.)

OR:
Non, il est impossible d'apprendre le chinois en un an ! (This
sentence contains the direct object after the infinitive.)

Il est facile d'apprendre l'italien. It is easy to
learn Italian. (The idea has not already been mentioned, and the direct object
is used.)

Adjectives that express a certain emotion require de before
the infinitive: content, désolé, furieux, heureux, triste

Je suis contente de vous voir. I am happy to see you.

Other adjectives require à before the infinitive:
agréable, pénible, terrible, amusant, intéressant, ennuyeux,
léger, lourd, lent, rapide, premier, dernier, prêt, seul

Il est prêt à partir. He is ready to leave.

In addition, when quelque chose is followed by an adjective,
de is inserted between the two. quelque chose d'intéressant
= something interesting

This article was used with permission from:
Indo-European Languages