In fact 'en' and 'eis' (I have not your Greeks fonts already, I have to download them soon, cause I feel the need to it
) are the same word.
Yes, 'en' and 'eis' are the same word (astonishing!). Firstly the choice of the case is obvious; the dative with 'en' to mark a position and an accusative with 'eis' to mark a sort of motion.
This difference also formed 'eis'. 'eis' origanally came form 'en' +'s'. This form 'ens' did evolve to 'eis'. The nu (n) dissapeared before a sigma (s)because off the laws of Greek fonetics. And if something dissapears, there must be a compensation for that in Greek fonetics (mostly so). So cause off the dissapearing off the nu (n) the vowel before it was lengthened from 'e' to 'ei = e+e'.
Here I gave you the fonetic analysis. When we look even closer and in comparation with other words, we see that sigma 's' is a suffix to denote a motion.
Conclusion: eis = en + s (suffix (signifies motion)) + the cases have the same difference.
In Latin we only have in+acc. in this text. So you can only see the difference by comparing with other texts or by seeing the forms in the contexts.
I have to agree with Benissimus on this: ' This is why so many things are lost in translation and it is important to read things in their original texts if you wish to grasp the original sentiments!'. The Italian have a proverb for this; traduttore - tradittore! (A translater is always a bit of a traitor!).