These words are called "determinantes indefinidos" in Spanish, because they talk about an unspecified quantity and they are frequently used in questions. The correct form of these "determinantes" are: "algún/alguno - alguna" > SINGULAR "algunos - algunas" > PLURAL Regarding the use of "ALGÚN/ALGUNO", ALGÚN is always followed by a noun. ALGUNO can't be followed by a noun. ·If it is before the noun, we use: "¿Tienes ALGÚN libro de aventuras?" - The common answer is "Sí, tengo uno" or "No tengo ninguno" ·After the noun we use the form "alguno": "Necesito un libro de aventuras, ¿tienes ALGUNO?" - We can use the common answers above or also "Creo que tengo ALGUNO" [The same goes for "NINGÚN/NINGUNO"] *Keep in mind that the rest of "determinantes" ALGUNA, ALGUNAS, and ALGUNOS have the same form before the noun and isolated. I hope your doubts are solved! ;)

Algún is used (without any of its endings) when referring to a singular situation or object. For example: algún día iré a México. It translates to "Some day I will go to Mexico." Algún vestido me tiene que quedar = "Some dress has to fit me." If you refer to a singular person the ending changes to "na" or "no." "Alguna" or "Alguno"