Spanish language Q&A Forum

This is a quote from kwiziq that is supposed to be explanatory, but it does not suggest a rule to know which adjectives have this form of ending. How are we to know which adjectives have this irregularity? Why can they not just follow the regular formula?

Hi.

This is quite a comprehensive list to refer to people from Europe and most of North, Central and South America. I notice though that “hondureño” is missing from the list of Central American nationalities. Perhaps it could be added? Also, countries in Asia, Africa and Australasia are listed in the corresponding exercise on countries but not mentioned here. Perhaps the two lists could be tweaked to make them match up? And finally, why not list “Escocia”, “Inglaterra”, “Irlanda” “ Irlanda del Norte” and “Gales” in the list of countries (companion exercise) since people from those places would rarely say - at least initially - that they’re from the “Reino Unido”?

Why is it  eres (ballerina) And not eres una (ballerina)

If the following is true:

Bear in mind that used as adjectives, the hundreds must agree with the noun they accompany: 

Había veinte mil trescientas personas en el concierto.

There were twenty thousand three hundred people at the concert.

Then why is the following the opposite?

Ciento is invariable and doesn't have to agree with the noun it accompanies, for example:

Ciento treinta y cinco mil casas

One hundred and thirty-five thousand houses

how do you know when to add the "que"  or is it interchangeable?

In the question

¡________ basura hay en esta casa!
Que mucha was an option offered that I chose and was marked wrong in favour of another option  cuánta.
Que mucha ...   translated in online dictionaries just fine.
Is there a difference ?