Ruy Lopez: Morphy Defense, Caro Variation

Hi!

I have played chess for more than half a year and still can't quite grasp the idea behind one of the most used openings, the Ruy Lopez. For Black, the following moves seem very logical

1 e4 e5

2 Nf3 Nc6

3 Bb5 a6

4 Ba4 b5

5 Bb3

- Black gets "free" pawn moves with tempo on the bishop. However, this is considered an inferior defense. Why? Is the pawn structure weak in some way or is the b3 square better for the bishop than the b5 square?

As you already noted, It's not clear that those pawn moves are "free." White often pushes a4 and gives black the choice of either giving up the a-file or pushing his pawn to b4 and leaving some weak squares. I'm not saying this is always bad for black, but you have to be aware of the possibilities.

There are some real advantages to having the bishop on b3. In the Italian game/Two knights, black often counterattacks in the center with d5!  the fact that the pawn attacks the Bc4 helps make it more forcing. With the bishop on b, this counterattack often loses its sting.  Also, White usually pays c3, which means the Bb3 can move to c2 and avoid exchange

There is nothing wrong with this variation. For most years 3..a6 was the main variation and it could be Blacks best move. But pawn moves are not necessarily good. As Bent Larsen put is: Positional understanding is to understand that pawns cannot go backwards. 

Or to use another frase: a threat is stronger than its executions. Black can play b5 whenever he wants. Why do it in the 4. move.