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.

A longer list of adjectives that require à or de before an infinitive can be found at 91. on French V.

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