On page 37 Duff shows you the two other feminine patterns for the first declension, ἡμέρα and δόξα. Notice that the plural forms are exactly the same as for ἀρχή. You get to the stem by removing the -α of the nominative singular.

- α nouns with vowel or ρ at end of stem- α nouns with anything else at end of stem
genitive ἡμέραςδόξης
genitive ἡμερῶνδοξῶν
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For these nouns ending in – α , what happens in the singular is determined by what you’re left with at the end of the stem after you’ve removed the – α . If the stem now ends with a vowel (most commonly ι) or ρ , then the word keeps alphas all through its singular endings. If it ends with any other letter, the noun is a kind of hybrid of ἡμέρα and ἀρχή – alphas and then etas in the case endings.

One important thing to notice: for nouns following the ἡμέρα pattern (alpha pure) – the genitive singular and the accusative plural look the same. This means that we now have to absorb one of the most important concepts in translating Greek: don’t decide too soon which form you have in front of you, before you’ve read through the whole sentence, so that you can decide which case makes the best sense in the context.