Can I capture en-pas?

The day a new chess player discovers en-passant is often a very significant day indeed. This new 'sneaky' move suddenly becomes a go to weapon, setting the player apart from their peers. However it can be overused, especially if the rule is only half understood.
As a junior tournament arbiter 'Can I capture en-passant?' is one of the most common questions I get asked.  Players will try and capture en-pas pawns in front of other pawns, pawns that have moved only one square, or just at some random moment in the game. Worse still, some will try and capture their opponents pieces 'en-pas' or try and capture pawns with their pieces in a similar manner.
However there is one situation where experienced players sometimes get a little confused, mainly because it is quite rare. It is where a pawn has advanced to the sixth rank, and then a pawn on the adjacent file pushes two squares ahead. On more than one occasion a player has looked at the board, looked at me and then started with 'Uh, can I uh, hang on, can I ...' I usually just smile and shake my head, or give a longer explanation if asked.
If you look at the game below you will see an example of this after whites 17th move. Of course both players in this game do know the rule, so the title question never came up. What did happen instead was that Caruana just demolished Radjabov in another 18 moves, and move to 4.5/5 in the Gashimov Memorial.

Caruana,Fabiano (2804) - Radjabov,Teimour (2726) [B31]
Vugar Gashimov Mem 2016 Shamkir AZE (5.3), 30.05.2016