Please help me follow this explanation.

Question

Please help me follow this explanation."We use the preposition "de" after the adjective..." by adjective do you mean facil/dificil?when the subject is "the thing" (el armario) - Can you expand please as to what "the subject being the thing" means? What thing? What does thing refer to here?"...not when the subject is "doing something" (montar el armario)."  Isn't the subject always doing something? I don't understand this distinction.All the examples are great and I can sense the pattern but I'd like something more concrete to be able to lock-in to the rule please :-)

Accepted Answer

Hola Tony
When we say the subject is "the thing" as opposed to "doing something" we mean this:
El armario es difícil de montar.
Here, if you analise the sentence, we have a subject which is a thing: el armario, so:
el armario = subject
es = verb 
However here:
Es difícil montar el armario. ( or what is the same: Montar el armario es difícil.)
Montar el armario (i.e. doing something) = subject
es = verb 
So we have two different kinds of subjects, one where we are talking about "a thing" and one where the subject is "an action with an infinitive" (= doing something)
I hope this makes you see the difference.
Here are other contrasting examples:

Este coche es fácil de conducir.
This car is easy to drive.
Conducir este coche es fácil.
Driving this car is easy.
Mi perro es difícil de entrenar.
My dog is difficult to train.
Es difícil entrenar a mi perro.
Training my dog is difficult.

Let us know if you are still in doubt.
Saludos
Inma