Diagonals

Just as rooks need open files, so bishops require open diagonals in order to take an active part in affairs. However, it is not always easy to find jobs for both of our bishops. If, for example, we have a lot of pawns on the light squares, then our dark-squared bishop will cooperate very well with them but the light-squared bishop will tend to be a problem. Black often has this type of difficulty in the Queen's Gambit. The bishop being a valuable piece, some attempt must be made to bring it into the game. So in the Queen's Gambit various plans have been developed for Black which involve the opening of a diagonal for the light-squared bishop.

On the whole there tend to be fewer problems with a fianchettoed bishop. But sometimes its scope may be limited by pawns, either our own or the opponent's. In that case we have to try to open the long diagonal so as to activate the bishop.

Of course, sometimes the bishop has to be developed to a relatively passive position. This may be necessary because we need to develop the rooks, and the best way to do that is to connect them. However, occasionally the a1-rook (or more rarely the h1-rook) can be brought into play without moving the bishop from c1 (or f1).

Here are a few useful guidelines, which can be very helpful in some (but not all!) situations. 
1) If a bishop is developed outside its own pawn chain, then pawns placed on the same-colored squares as that bishop will not hamper the other bishop either. So we can have both bishops in active posts. White follows this sort of strategy in, for example, the Torre Attack. 
2) If a bishop is exchanged for a knight, you should try to place your pawns on the same-colored squares as the bishop that has been exchanged (Capablanca's rule). This will help avoid any weakness on these squares, and also free the other bishop. 
3) There are two contrasting strategies for combating an opposing bishop: 
a) Post your pawns on opposite-colored squares to the bishop. This is very effective in the endgame, but it is sometimes a disadvantage in the middlegame, as the opponent is allowed to control a lot of squares and can use them to penetrate with his pieces. 
b) Set up a barrier of pawns on squares of the same color as the bishop. This is often good in the middlegame, but care has to be taken in the endgame to ensure that the bishop cannot attack your pawn chain from the rear.

I don't understand 2) Capablanca's rule against 3a) the opposite rule.