Pitman's Commercial Spanish Grammar (2nd ed.)

The Definite Article in Spanish is

El before a masculine[7] noun singular
La  "       feminine[7]    "    "

Footnote 7: Spanish nouns are all masculine or feminine. There are no neuter nouns.

as --

El hombre (the man) -- La mujer (the woman)
El libro (the book) -- La pluma (the pen)

El recibo (the receipt) -- La cuenta (the account)

Los before a masculine noun plural

Las    "     feminine    "    "

as --

Los muchachos (the boys) -- Las señoras (the ladies)
Los géneros (the goods) -- Las facturas (the invoices)
Los lápices (the pencils) -- Las cartas (the letters).

The Indefinite Article is --

Un before a masculine noun singular
Una     "   feminine    "     "

as --

Un amigo (a friend) -- Una amiga (a lady-friend)
Un padre (a father) -- Una madre (a mother)

The Indefinite Article has no plural, but the Spanish plural forms "unos" (masc.) and "unas" (fem.) translate the English words "some" or "any," as Unos hermanos (some brothers), Unas hermanas (some sisters), Unos tinteros (some inkstands), Unas mesas (some tables). (The Spanish words "Algunos," "Algunas," are also used for the same purpose.)

NOTE 1. -- Before a feminine noun singular commencing with a or ha use El and Un instead of La and Una if such nouns are stressed on the first syllable, as El águila (the eagle), El agua (the water), El alma (the soul). (The plural is regular, as Las águilas (the eagles).)

NOTE 2. -- The Definite Article has a "neuter form" which is Lo. It cannot be used before a noun but before other parts of speech used to represent an abstract idea, as Yo amo lo bello (I love the beautiful, viz., all that which is beautiful), Lo sublime (the sublime, viz., all that which is sublime).