Supposedly, according to general consensus, if black were to play 1.e4 e5 2.Nf3 d6 3.d4 Nc6 4.Bb5 Bg4 5.dxe5 dxe5, then that would be a mistake. However, the novice brain that I have can't see how to get the advantage as white. Can someone help me?

Well, looking at this position really quickly, b x knight ch. pawn x b then q x q followed by the win of the king pawn looks good for white.

surely you can't win the king pawn because of this:

 

wlapre wrote: Well, looking at this position really quickly, b x knight ch. pawn x b then q x q followed by the win of the king pawn looks good for white.

---

That line fails to a ....Rd1# ! Its better for white to play either 4dxe5 right away or after 4Bb5 Bg4 5d5 !

I believe in the philidor the best option for black at move 3 is 3....Nf6

haha well i thought i saw the answer for a second but black checkmates!  white can still double isolate blacks c pawn but black gets the two bishops, and white's pieces are awkward.  6.0-0 is probably best.  Technically i think 5.dxe5 is a mistake.  When you played Bb5 you transposed to the old steinitz variation of the ruy lopez, and d5 is the book move. 

thanks ppl!

According to chess assistant 4Bc4 and 4Bb5 both score 75% against 3....Nc6, while 4dxe5 scores 79%. The very fact that white has 3 moves scoring over 70% against 3....Nc6 pretty much condemns the move imo. 3...Nf6 is the highest scoring for black at 44% while 3...Nc6 only scores 26%.

how about Qd5 after black plays dxe5?

This is my faverite line of the Philidor's Defence ,1 e4 e5, 2 Nf3 d6,3 d4 f5! I have had lot of good games with it, give a lot of problems to white he does not  kknow what to do with it Thanks