I am not entirely sure which circumstances arise where you are not permitted to use the "en passant" move.   Any feedback would be appreciated.

When the opponent moves his pawn two spots up right next to your pawn you can take it. You have to do it on your first move. 

thanks tactician,  can you also make that move at the very edge of the chess board?

Yeah,

One time you would not be able to make that move is: You are not allowed to expose your king to check by moving your pawn to take the opponents pawn "en passant".

yes this is fine