Using ser fácil/difícil + infinitive vs ser fácil/difícil de + infinitive

Confused about fácil/difícil + infinitive and  fácil/difícil + de + infinitive in Spanish?

It's often difficult to know whether to say:

es fácil/difícil de hacer or es fácil/difícil hacer

We can say both. Have a look at these examples:

We use the preposition "de" after the adjective when the subject is "the thing" (el armario), not when the subject is "doing something" (montar el armario).

Here are other examples with the adjective difícil:

This is true for all adjectives. Here are more contrasting examples:

It is common to see muy and tan in front of the adjective in this construction. For example:

See also Using [verb] + tan + [adjective/adverb] + que to say "so... that..." and Exclamatory sentence in Spanish: qué + noun + más/tan + adjective