Ancient Greek Accentuation: the article

The forms which begin with vowel (ὁ, ἡ, οἱ, αἱ) are proclitics.

All other nominative and accusative forms are oxytone.

All genitive and dative forms are perispomenon.

Num Case Masc Fem Neut
Sg Nom ὁ ἡ τό
Acc τόν τήν τό
Gen τοῦ τῆς τοῦ
Dat τῷ τῇ τῷ
Pl Nom οἱ αἱ τά
Acc τούς τάς τά
Gen τῶν τῶν τῶν
Dat τοῖς ταῖς τοῖς

These rules also hold for the dual:

Num Case Masc Fem Neut
Dl Nom/Acc τώ τώ τώ
Gen/Dat τοῖν τοῖν τοῖν