Question

The use of a, an, or the is a signal that a______ is coming

Answer 1

The use of a, an, or the is a signal that a noun is coming.
In the English language, only nouns are preceded by articles. A is used when a noun starts with a consonant (a book, a table, a bottle); an is used when the noun starts with a vowel (an apple, an umbrella, an avocado); the is used when you are referring to a particular thing (The boy I just met is called Mark.)

Answer 2

A, an, or the (which are articles) introduce a noun.