What is the difference between "ser difícil de" and "ser díficil"

Question

Hi, InmaI translated the sentence "it would be very difficult to determine it." into "sería difícil de definirlo", and the correct answer removed the "de".But when I translated the sentence "that will be hard to forget for tango lovers" into "que será difícil olvidar para los amantes del tango." the correct answer asked me to add a "de" after "difícil". I'm so confused now.SaludosWenli

Accepted Answer

Hola Wenli
The answer is in this Kwiziq lesson, precisely about when to use fácil/difícil  vs fácil/difícil de 
Let us know if you still have some doubts about it after having a look at the lesson. 
(You should have found a link to that lesson in this exercise, we'll add it shortly) 
Saludos
Inma