2. e5 in the Robatsch (Modern) Defence Since the modern defence is a hypermodern opening, it focuses on controlling the center through pieces not pawns. Therefore, I find it an advantage to block these pieces by pawns. So, I defend against the modern defence like this ..

[fen ""]
1. e4 g6 2. e5


The idea is to block the bishop that is coming to g7 with 2...Bg7, and to prevent the knight from going to f6 and thus prevents castling kingside.

Now, if Black tries to attack the pawn, this happens ..

[fen ""]
1. e4 g6 2. e5 f6 3. d4 fxe5 4. dxe5 d6 5. Nf3


If Black takes the pawn, then white is clearly ahead in development. If he doesn't, then he can't deploy his pieces effectively.

Anything wrong with this plan ?
OK, not really an answer as it's looking entirely at what I, a complete Pirc addict, would play, there may well be better lines. But unfortunately you can't stick a pgn in a comment, so here's a few minutes work with Fritz to see what it thinks - after all it's a much better player than I. Numbers are in units of pawns, negative numbers mean advantage to black. Note the numbers appear in the box below the board once the last move of the line is played. Lines quoted are from the top 4 selected by Fritz, biased where appropriate by the comments above, and in each case I left it a few minutes for the evaluation to settle down.

[Event "?"]
[Site "?"]
[Date "2015.09.21"]
[Round "?"]
[White "????"]
[Black "????"]
[Result "*"]
[ECO "B06"]
[PlyCount "13"]
[FEN ""]

1. e4 g6 2. e5 Bg7 3. d4 d6 4. f4 c5 5. Nf3 (5. dxc5 Qa5+ 6. Qd2 {-0.21}) (5.
c3 dxe5 6. fxe5 cxd4 7. cxd4 {-0.32}) (5. d5 dxe5 6. fxe5 Bxe5 {-0.48}) 5...
cxd4 6. Qxd4 Nh6 (6... Nc6 7. Qd2 {-0.70}) 7. Bb5+ {-0.77} *


So not disastrous for white but according to Fritz at least he has thrown away his first move advantage.

Oh, and the point about the tempo is that by playing e5 before black has played Nf6 is that the knight can wait snuggly at home, thinking about where to go. If you wait until after Nf6 has happened black has to "waste" a move, after which you can further your evil plan. Compare the above with

[Event "?"]
[Site "?"]
[Date "2015.09.21"]
[Round "?"]
[White "????"]
[Black "????"]
[Result "*"]
[ECO "B09"]
[PlyCount "14"]
[TimeControl "1"]
[FEN ""]

 1. e4 g6 2. d4 Bg7 3. Nc3
d6 4. f4 Nf6 5. Nf3 c5 6. Bb5+ Bd7 7. e5 {+0.07} Ng4 *