Abarim Publications' online Biblical Greek Dictionary
ο - η - το
In Greek the definite article follows the gender, number and case of the noun it sits in front of, and that leads to a neat array of forms in which it occurs. All these words can be translated into English as "the":
Note that without accents (as Koine Greek was originally written) the forms ο, η, οι and αι are identical to four forms of the relative pronoun ος (hos). The feminine nominative definite article is additionally spelled the same as the comparative particle η, (e), meaning or.
Together with the enclitic particle δε (de), which indicates a mild objection or simple transition, the various forms of the article may form demonstrative pronouns like τηδε (tede) or ταδε (tade), and so on. These words mean "these [things]" or "this [item]" and refer to a thing or person last mentioned (Luke 10:39) or introduces whatever follows (Revelation 2:1).