It is not so important if you say τη or την

Oh, many thanks Francesco! The Greek endings need too much effort!

Let's start with the Accusative Singular articles (both Feminine and Masculine). You use the form with the -ν ending (Fem. την, Masc. τον) when the following word starts with a vowel or the consonants κ, π, τ, ξ, ψ, γκ, μπ, ντ. If the following word starts with a different letter you use the form without the -ν ending (Fem. τη, Masc. το). As for οι (without the tonic stress) and αἱ, they are Nominative Plural articles, not Accusative Plural articles. Oι is both the Masculine and the Feminine Nominative Plural article in Modern Greek. Aἱ was the Feminine Nominative Plural article in Ancient Greek and it is not used nowadays (except when archaic denominations are used on purpose as for example αἱ Αθήναι for η Αθήνα or αἱ Πάτραι for η Πάτρα). The Feminine Accusative Plural article is τις.