Fácil (de) aprender

[fglorca]

Are all the sentences below correct?
When should we include the preposition de? What is the rule?

• Es fácil aprender a jugar al fútbol.
• El fútbol es un deporte fácil de aprender.

• Es fácil leer ese libro.
• Ese libro es fácil de leer.

• Practico mucho el fútbol, pero es difícil mejorar.
• El fútbol es difícil de mejorar.

Many thanks in advance.

[aleCcowaN]

everything

the rule is what you did with those sentences

[wrholt]

If you haven't noticed it already:

Es fácil/difícil leer ese libro. = Reading this book is easy/hard.
Ese libro es fácil/difícil de leer. = This book is easy/hard to read.

But:

Es fácil/difícil leer esos libros. = Reading these books is easy/hard.
Esos libros son fáciles/difíciles de leer. = These books are easy/hard to read.

[fglorca]

Many thanks, aleCcowaN and wrholt.
Is the following use of de in "difícil de entender" grammatically incorrect so? Thanks again!

Juan puede quedarse en las discotecas hasta las tres de la madrugada. Los padres de Juan son muy tolerantes. Es difícil de entender su permisividad.

Should it just be:
Es difícil entender su permisividad.

[AngelicaDeAlquezar]

"Difícil entender" sounds better.

[wrholt]

Sentences 1 and 2 are grammatically identical, even though the word order is different:
1. Es difícil entender su permisividad
2. Entender su permisividad es difícil.
'
Subject: entender su permisividad (noun phrase = infinitive[noun] + direct object)
Predicate: es difícil (copula "es" + predicate adjective "difícil")

Sentence 3 has the same meaning as sentences 1 and 2, but the grammatical structure is different:

3. Su permisividad es difícil de entender
Subject: su permisividad (noun phrase)
Predicate: es difícil de entender (copula "es" + predicate adjective phrase "difícil de entender" = "difícil" + adverbial phrase "de entender" = preposition "de" + noun "entender").

Note that "entender" is a transitive verb, and it requires a direct object. In sentence 3 "entender" does not have grammatical direct object; however, the subject "su permisividad" is the logical direct object.

Sentence 4 does not sound good.

4. Es difícil de entender su permisividad.

The issue with sentence 4 is that it is too similar to sentence 1. Listeners can't decide whether "entender" is supposed to be part of the subject "entender su permisividad" or whether it's supposed to be part of the predicate "es difícil de entender", and it CANNOT fill both roles at the same time. So speakers/writers avoid sentence 4.