When not to move a wide pawn?

Hello, 

I am new to chess. Well, I am 35 and learned when I was about 8, but obviously it is not something I have spent a lot of time on. However, I would like to be better, and I am looking at openings as a way of improving my game. 

So the point ofmy post is, in the Ruy Lopez (and this applies to a lot of openings), white has moved his bishop to attack a black knight. 

I know there are several highly rated moves to follow on from this such as d6, nge7, nf6, nd4, bc5, f5, but why not a6, putting pressure on the white bishop and forcing it to move? 

And the same question for the other side. If white moves a bishop or queen to say, g6, why should black not advance the h7 pawn (I guess I can see some milage here if you want to castle on that side, you may not want to advance a pawn changing your pawn structure too early?)?

a6 is pretty much the main move there.

It's always hard to find the right moment to challenge the bishop. If it's a queen bishop, pinning a king's knight, you usually don't want to challenge te bishop with pawns, because it significantly weakens the king's pawn structure, but simply break the pin by moving the queen or putting a bishop on e2 (e7).

A king's bishop pinning a queen's knight is a bit different. If you castle kingside, then challenging the bishop with pawns isn't as dangerous. The downside to this is that the bishop may be after ruining the pawn structure by creating a doubled pawn on the c-file.

There are many nuances like this you want to consider when you are thinking about solving the problem caused by the pin.

a6 is by far the most common move there. a6/h6 to attack a Bishop that is attacking and in some cases pinning a Knight is quite frequently a good idea. It's called 'puttng the question to the Bishop' and can determine a lot of how the game will proceed.

Usually you shouldn't play h6 to attack a Bishop on g5 if they can take your knight with the Bishop and you have to recapture with the g7-pawn. This creates some weaknesses. However, if there are piece recaptures available it is quite frequently a good idea. This also creates a flight square (luft) for a castled King on that side. However, if you are castled on that side and your opponent is not, beware of making weakening pawn moves like h7-h6, as your opponent may reply h2-h4 sacrificing the Bishop to open the rook file toward your king. Just something to keep in mind!

Ahhhh thanks. 

Just read further into the book and it is indeed listed a couple of pages later as the Morphy's Defence.