I remember learning the rule for when to use cualquier, cualquiera, and cualesquiera, but was always confused about the exact differences. When should each be used, and what role do number, gender, and word placement play? Specifically, what should the translation be in the phrase "any of us," and why is that the proper choice?
"Cualquiera" can work as an determinative adjective (used with a noun) or as a pronoun.
In this site it is explained its use as a determinative:
It is the singular form and it appears before the noun, which can be both masculine and feminine:
It is the singular form and it appears after the noun, which can be both masculine and feminine:
So, use cualquier or cualquiera when you any to say "any", one single thing. "Cualquier" is the shorted form, and "cualquiera" is the full form. Just pick any of them and be careful with the placement.
It is the plural form and it appears after the noun, which can be both masculine and feminine:
However, it is quite spread the use of "cualquiera" in plural nouns, but this is not correct:
Cualesquiera is the real word.
Cualquier and Cualquiera are short forms.
You can use Cualquier with any word if it's masculine and before the noun.
You must use Cualquiera if is feminine.
A common error is use "cualquier" in a feminine case, it's really common in oral language, but it's wrong.
The same happens with first (primero)