When to use "à" and "de" after adjective (e.g. impossible, difficile)? [duplicate]

Question

I found the following sentences:

Il est impossible de penser à cela.

Difficile à dire.

C'est difficile de choisir.

Ces choses sont impossibles à comprendre.

These sentences are not coherent on the proposition of the adjectives, namely à and de.

But I don't understand when to use à and when to use de. How can I use them apart correctly?

Answer 1

After an adjective, you can use à when this adjective is related to some kind of ability, attitude, or imminent action.

De is used to express feelings, stats of mind, and obligation.

You can see my source and some eg. in the following link: https://www.lawlessfrench.com/grammar/adjectives-with-prepositions/