The full pattern of the definite article is on p36 of Duff. Notice for the feminine, ‘feminine’ vowels -η and –α crop up in the endings (think of the endings of names such as Maria and Zoe). The genitive singular is the one to notice, introducing a different kind of ending from the masculine and neuter patterns. Notice too that the genitive plural always ends in –ων; this holds for every pattern of noun in every declension.
The definite article is an extremely important pattern; devote time to learning it, as it will help you enormously with working out what’s going on with unfamiliar nouns in a sentence. It’s worth spending a few minutes every day reciting it:
The first declension contains only feminine and masculine patterns. The first pattern we’ll consider is the one that follows most closely the pattern of the feminine definite article: feminine nouns that end in -η and run like ἀρχή.
Notice the iota in the dative endings, both singular (as iota subscript, under the final letter) and plural; this is a clue that runs through all the noun patterns. Genitive plural ends in -ων, as for all noun patterns.
In order to get to the stem, remove the –η from the nominative singular.