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. The definite article, in all its many forms, is the most frequently occurring word in the New Testament, with 20,307 occurrences. That comes down to a little over 2.5 times per verse, so we understandably don't publish a concordance of this word.
Often the Greek article can be translated into English as "the", but frequently the Greek uses the article where English wouldn't (for instance in front of names: "Abraham begat the Isaac" — Matthew 1:2):
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 οδε (hode) or 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) and occur 11 times in the New Testament; see full concordance. This demonstrative pronoun corresponds to the demonstrative pronoun ουτος (houtos), meaning this or that.