A short course in the DUAL (esp. Plato Ap. 20a-b):
1st decl. 2nd decl. 3rd decl
-α (nom/acc) -ω -ε
-αιν (gen/dat) -οιν -οιν
Definite article (all genders)= τώ... τοῖν
Verbs: primary, 2nd & 3rd person = -τον (act.) -σθον (mid./pass.)
as in ἐστόν, 'they two are'
secondary (esp. aorist and optative) = -τον / -σθον for 2nd person
-την, -σθην for 3rd person. as in ἐγενέσθην, 'they two were'