The (definite) article in Greek is, of course, extremely important – that’s why it’s one of the first things new students learn.
Tip 1: remember that masculine and neuter forms are often identical (making memorisation easier): in the genitive and dative both singular and plural.
Tip 2: apart from the masc and fem nominative singular and plural (i.e. only four forms) the relative pronoun is the same as article, but without the τ (and it always has a rough breathing).