es dificil entender / es dificil de entender

[rpicheme]

I looked for a previous thread on this topic, but couldn't find one . . . if there is one and I missed it, of course feel free to send me in that direction.

Anyway, I'm confused about whether a preposition is necessary in sentences that start "es dificil (a verb infinitive)" (or for that matter "es facil . . ."). What are the rules here?

Thanks in advance for the help.

 

[Yyrkoon]

It depends on what you want to say, if you say
"es difícil de entender"-->You are talking about a special idea or concept
"es difícil entender (lo que dices)--->always is difficult to understand what you say.

It's more or less like your "the" you can say I go to the university reffering to a special one, or you can say I go to university as a general thing in your life, isn't it?