Premature Pawn Advance versus the Pirc
One of the key considerations in the Pirc (e4 d6 d4 Nf6 Nc3 g6) defense is what will happen when white plays e5. Often the knight on f6 needs a good home and even when the knight moves e6 can be an issue. On the other hand, if white plays e5 too soon black can often pressure white's center effecively gaining equality with ease. Here is a recent game of mine
Black has three options, take the pawn then jump the knight, jump the knight, take the pawn exchange queens and then jump the knight.
In addition to the game line we can also look at the same early e5 in positions where black has played O-O or c6 instead of a6.
The first thing to note is that a6 leaves d5 undefended unlike c6. From d5 the c7 pawn has only the queen guarding against a nasty knight fork. Thus dxe5 dxe5 Qxd1+ Rxd1 leaves black in a quandry since there's no good defense against Nd5.
Now we know trading queens removes our sole guardian of c7 which is a big weakness in this line. We can also try and exchange and then block white's queen. So dxe5 dxe5 Nfd7 but now the Pirc's other enemy can be played e6! and black's position gets ugly after fxe6 and Ng5
Of course playing dxe5 dxe5 Ng4 Qxd8+ Kxd8 Rd1+ Bd7 still leaves black in a mess. Thus we come to Ng4 targeting e5 and white's precious bishop. e6 obviously does nothing here and Nd5 is quickly met with Nxe3. White can continue by moving the bishop or trading on e6 all those lines are equal for black.
Thus we can see that the early e5 advance can be exploited by black but it must be done in a careful manner. Lastly we can see that if black prevents the issues on d5 by playing c6 or O-O that this allows 6...dxe5 without the same problems although the queen exchange is still bad and Ng4 still ends up being best