There is a third neuter article, lo.

This article has no plural. It has a wide range of use before adjectives and certain other parts of speech to make them into nouns:

lo verdadero — that which is true
lo posible — that which is possible
lo mío — mine, that which is mine

(Compare this with la mía, which would refer to an antecedent feminine noun.)

lo difícil — the difficult thing
lo importante — the important thing

Notice that lo is used where the referent idea is not some specific thing but rather an abstract situation or collection of unidentified things:

Me pasó a mí lo mismo. — The same thing happened to me.
Lo del jefe ya se solucionó. — The problem regarding the boss is already solved.

This same neuter gender also shows up in the neuter demonstrative pronouns, esto, eso and aquello, more or less corresponding to “this, that and the other.”

Alcánceme eso. — Hand me that (thing).
¿Qué es esto? — What's this?
No quiero discutir aquello del otro día. — I don't want to argue about the issue of the other day.

These neuter demonstrative pronouns, like lo, the neuter article, also never occur in the plural and never refer to a gendered word but rather to unidentified or abstract situations or things.

In the lessons you will see how useful neuter can be in conversations.