Difícil vs difíciles
 

Listening to a language learning podcast, even though the focus was not on the use of difícil or difíciles, I did notice the teacher saying the following when giving example sentences:

Rules are difficult to understand
Las reglas son difícil de entender

I am questioning why did she not say son difíciles de entender?

Chileans are difficult to understand
Los chilenos son difícil de entender

I am questioning why she did not say son difíciles de entender?

How would you have said it?

Are both ways commonly used or is one of them incorrect?

Side note:
I think this teacher is from Colombia. I found it interesting to also hear her using the word “computador” instead of “computadora,” which I learned in Mexico, or “ordenador,” which I learned in Spain. This just shows how much variation there is from country to country and region to region that makes the Spanish language so rich!!

In both cases I'd say "son difíciles de entender", and any other audacity is wrong. :D

Seriously, I don't know if this usage is accepted in other regions, but probably the teacher believes "difícil de entender" is an adverbial construction, and since adverbs don't agree in gender or number with the noun, "difícil de entender" would be correct.

However, "difícil" is always an adjective and "de entender", here, is a complement to this adjective. "Difícil" is not a word like "rápido", which can be an adjective or an adverb (meaning "rápidamente"), depending on the sentence, so I think she is wrong, unless this is regional usage considered correct by their local Academia. ;)

Thank you, AngelicaDeAlquezar. I'm glad I questioned her usage so I won't learn it wrong.

That clears it up for me.