Distinguishing gender in A-Declension

Hi,
I am stuck in the A-Declension for feminine nouns. Any help would be appreciated! I felt like I was on roll in the A-Declension for feminine nouns! But then I got to this point:
“The gender of a noun is indicated by placing it after the corresponding form of the definite article, as αγορα, ας, η, where η signifies that αγορα is feminine.”
This has been confusing me, and there’s no further explanation or example. These are my questions:
Does “it” refer to the gender or the the noun. And when they say the corresponding form of the definite article, how do I know what the “definite article” is? And do ας and η go at the beginning of the word or end (i.e. ας αγορα or αγορας or η αγορα or αγορη)? It makes sense that they would go at the beginning as η αγορα, because the conjugation doesn’t show words that end with α suddently obtaining a η at the end of the noun for any of the cases, but I want to double check.
Thanks!

  1. it refers to the noun.
  2. The conjugation of the definite article by gender, case and number is present at the beginning of
    every Greek Grammar textbook. The singular feminine article in the nominative case is ἡ.
  1. ᾶς is the singular feminine ending of the genitive case in the A declension. τῆς is the singular
    feminine article of the genitive case. In vocabulary lists and dictionaries, this is the standard way
    of writing the head word: noun (sg. nom.), gen. ending, definite article (sg. nom.). So ἀγορά, ᾶς, ἡ
    corresponds to the noun (sg. nom. in A declension), its genitive case ending, and the sg. nom. fem.
    definite article.

Thank you!