Abarim Publications' online Biblical Greek Dictionary
There are two different prepositions spelled εν, namely εν (en), meaning in, and εν (hen), meaning one. The latter is the neutral form of the cardinal number εις (heis), meaning one.
The preposition εν (en) appears as leading element of a colossal array of words. It roughly works the same as the English preposition "en-" (as in words like: entrance, enthuse, enslave), although in Greek it's much more commonly and broadly used than in English.
It's used whenever we're in a place or building, on an elevation, amidst a multitude, in someone's presence, at someone's feet, enveloped by something, in a certain time period, in a certain situation, under a certain condition, in a certain way, under a certain influence, with the help of something.
Our preposition generally means in, at or by and expresses a situation either inside something or endowed with something, but always at rest — this in contrast to the more motive prepositions εις, eis, meaning into, and εκ, ek, meaning out of.
In the rare cases in which this preposition is used with verbs of motion, the use of εν (en) implies that we're looking at a situation that results from a motion inward, in stead of the actual movement.