Abarim Publications' online Biblical Greek Dictionary
επι
The preposition επι (epi) means on, upon or unto. It survives in English in words such as epilogue (upon, or after the main story), epidermis (upper skin) and epicenter (the point on the earth's surface directly above an earthquake).
The difference between επι (epi) and the similar ανα (ana), is that the latter emphasizes absorption, whereas the former emphasizes arrival at a boundary.
Our preposition is used in a great variety of ways, some implying rest (being physically located upon or logically completing a statement) and others implying motion (moving onto a location or toward a situation or understanding). It many provoke the genitive, dative and accusative cases, depending on context.
Our particle may reflect an overbearing authority or power, suggestive advice or motivational encouragement. It may even serve to indicate repetition (over-and-over), or as intensifier (above-and-beyond), like our prefixes "extra-" (extraordinary) or "super-" (supersonic).
Our particle επι (epi) is part of an enormous array of compound words, but also occurs independently in the New Testament a total of 900 times; see full concordance.
επει
Derived from the above, the conjunction επει (epei) ties two clauses together, mostly of time (when) and motivation (because, since). It occurs 27 times, see full concordance, and from it derive:
- Together with the curious particle αν (an), which expresses a mild conditional or wish: the conjunction επαν (epan), meaning when it may be, whenever or as soon as (Matthew 2:8, Luke 11:22 and 11:34 only).
- Together with the adverb δη (de), indeed or truly: the conjunction επειδη (epeide), which demonstrates motive: since now, inasmuch as. It occurs 10 times, see full concordance, and from it in turn comes:
- Together with the particle περ (per), wholly so: the conjunction επειδηπερ (epeideper), which means the same as the previous but with an emphasis on the surety: since very wholly truly so. This flowery conjunction occurs in Luke 1:1 only, as the first word of the Lukan gospel, where it clearly conveys satire: "Since so many of the best and brightest have done their utmost to accurately tell the story of Jesus, here's my version, my dear Theophilus, which is properly researched and actually drawn from the accounts of eye-witnesses."
- Again together with περ (per), wholly so: the conjunction επειπερ (epeiper), since indeed so, since wholly so (Romans 3:30 only).