The use of "de" before a word that seems unnecessary
In the sentence, "Es difícil de explicar" and others, I am confused by what seems to be a superfluous "de." If explicar means "to explain," why stick in the "de?" This comes up often and I want to know the "why" of it so perhaps it will help me remember.
3 Answers
There are two constructions which express exactly the same but are grammatically very different:
1) Es difícil explicar eso.
2) Eso es difícil de explicar. (or less frequent: Es difícil de explicar eso)
In the first one, the subject is "explicar eso", as in "explaining that", and then "difícil" is a kind of object which in Spanish is called "atributo". This sentence is similar to "Es difícil la lección", but using a phrase with an infinitive. This sentence is like "Explaining that is hard", which is Spanish goes like "Is hard explaining that".
In the second one, the subject is "eso", and "difícil de explicar" is the object called "atribute". What happens here is that the adjective "difícil" takes its own complement, like in "hard-to-explain". This sentence is like "That is hard to explain". While English requires "to" in these cases, in Spanish "de" is required.
What a good question!
Very difficult question to answer. Pregunta muy difícil de contestar.
Answering is difficult. Contestar es difícil.
Some examples:
No es fácil hablar de ese tema.
No es fácil hablarlo.
No es fácil hablar (hay mucho ruido).
El examen no es fácil de aprobar.
No es fácil aprobarlo.
No es fácil aprobar (la asignatura).
Un lenguaje no es tan simple de aprender.
No es tan simple aprenderlo.
No es tan simple aprender (cualquier cosa susceptible de aprenderse)
Lo que dices es difícil de asimilar.
Cuesta asimilarlo.
Cuesta asimilar (no tengo mi mente muy despierta)
Esta situación no es fácil de llevar.
No es fácil llevarlo.
No es fácil de llevar (siempre hay excepciones). 
This explanation is necessarily convoluted, but I think I got it!
Thank you so much for taking the time to help.
