Learn Spanish (Latin America): Puedes guardar cualquier pluma en este estuche. - You can keep any fountain pen in this pencil case.

The word cualquiera drops its final -a and becomes cualquier whenever it comes directly before a singular noun. This happens with both masculine and feminine nouns, so it's always cualquier pluma (feminine) and cualquier estuche (masculine).

Could I say Puedes guardar alguna pluma instead? How does the meaning change?

You could, but it changes the message. Alguna pluma means "some pen" or "a pen" (referring to an unspecified one). Using cualquier pluma emphasizes free choice—it literally doesn't matter which pen it is, absolutely "any" single one of them will do.

Doesn't guardar mean "to guard"? Why does the translation say "keep"?

Guardar is a bit of a false friend! While it can mean "to guard" or "to protect" in some contexts, its most common everyday meaning is "to put away," "to store," or "to keep" something in a specific place. Here, it describes putting the pen away in its case.

Is pluma the standard word for "pen" in Latin America?