=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 " "