Abarim Publications' online Biblical Greek Dictionary
The comparative particle η (e) basically means "or", but with some additional tricks up its sleeve. It may show up twice in one sentence, and then assumes the disjunctive comparative function of "either... or" (Matthew 6:24). It comes equipped with a slight tone of inquisition and suggestion: "or [wouldn't it be that]...?" (Matthew 9:5), or even confirmation of denial: "or [as you know there isn't]...? (Matthew 7:9)."
Our particle may serve to introduce a whole comparative clause, where in English would be used the word "than" (Matthew 10:25) or alternative (rather "than", Matthew 18:3). Together with the adverb of time πριν (prin), meaning "before" it means "sooner than" (Matthew 1:18). Combined with the conjunction αλλα (alla), meaning "but", our particle forms something like "unless" or "except". Together with the copulative και (kai), meaning "and", it forms "and also" or "and even".
This particle occurs 355 times in the New Testament, see full concordance, but note that it is identical to the feminine nominative single definite article ("the"), and the nominative and dative single forms of the relative pronoun ("that").