Indefinite choice (cualquiera, cualquier)

"Any" Noun: Cualquier

When you want to say "any" to express a free choice from a category (like "any pen" or "any stroke"), use the word cualquiera.

However, there is a strict rule: when this word comes directly before a singular noun, it drops its final -a and becomes cualquier. This happens with both masculine and feminine nouns!

Any ballpoint pen is useful for drawing a thick line.
Cualquier bolígrafo es útil para trazar una línea gruesa.
I am looking for any fountain pen that makes a thick stroke.
Busco cualquier pluma que haga un trazo grueso.
Any calligraphy improves if you use a fine stroke.
Cualquier caligrafía mejora si usas un trazo fino.

"Anyone" or "Any One": Cualquiera

When the word stands completely on its own—replacing the noun rather than describing it—it keeps its full form: cualquiera.

You will often see it used to mean "anyone" in general, such as when saying that a skill is accessible to all:

Anyone can learn calligraphy with a fine fountain pen.
Cualquiera puede aprender caligrafía con una pluma fina.

You also use the full form cualquiera when referring back to a noun that was already mentioned, meaning "any one" (of a specific group):

If there is not a ballpoint pen in the pencil case, you can use any one.
Si no hay un bolígrafo en el estuche, puedes usar cualquiera.