@hampy (Now, the following morning)
Keep in mind that, as has been previously noted or alluded to by other persons in this thread, these "phases" are more conceptual in nature than anything else. They are the result of learned and historical attempts to analyze the game of chess and understand it in order to extract certain rules that might be applied in the age-old quest to find the best move in all situations and, in some small fashion, stay closest to the "truth" of the game and evolving situations as possible.
More concretely, as applied to my preceding post about the "opening phase" of the game, this means that it and its particulars are offered as "useful generalizations". As such, one can easily come across (or point to, as is often done in these threads) situations that do not exactly match all that has been described or explained and this, often anecdotal evidence, will be presented as ample evidence to discredit anything and everything one might have written in the hopes of being of some concrete use to someone. So, let me say in advance, loud and clear: the "opening" and "middlegame" are sometimes not separated by as sharp a line of demarcation as I have painted the picture of above. Opening play is often less clearly strategic in nature, meandering through tactical considerations designed to obtain a quick victory or advantage so that it can become a difficult and somewhat arbitrary exercise to say: here the opening ends and middlegame begins.
Practically, however, such philosophical considerations are of almost no use to the beginning players like your son. So, my advice to you would be not to pay much attention to these types of trivial and anecdotal considerations and stick with the big picture. As your son improves and his understanding of the game evolves, these distinctions will eventually find their way into his mind or someone will point them out and he will be in a position to use that information rather than be discouraged by it. For the moment, let's just take it for granted that even though one may find "preliminary skirmishes" in the opening phase of the game, the decisive battle for supremacy belongs to the middlegame. ;D
So, getting back to the middlegame, and keeping in mind that I am trying to give you useful generalizations, you might say that the middlegame is that period of the game wherein forces have been deployed in quickest and most economical fashion possible, for maximum effect, according to the game's general "opening strategy" and the clash of wills is about to begin with, as a result, clear material and territorial consequences. In other words, the fight is about to begin and one can expect a loss of blood and casualties as well as shifting territory! This is especially true and recognizable in "open" games (generally 1.e4 games) where one or two central files have lost their pawns and provide the players with highways (files and diagonals) into enemy territory.
If one had to choose a defining characteristic of the middlegame, we could certainly do worse than to say: tactical considerations are ever-present, in this phase of the game. Players are constantly jockeying to come up with a position whereby pieces will be so harmoniously and advantageously placed that some form of simple or more complex tactic will result in an advantageous exchange or series of exchanges, leaving one with more or better material or a position that gives one more winning chances than the opponent.
Typically, these tactical forays are not too complex and consist of: pins, skewers, double-attacks, knight-forks, pawn-forks, deflection or removal of defender and checks, obviously. This is why one must absolutely study at least basic tactics when one hopes to improve even a little bit, in chess. Because once one gets out of the opening and is searching for a useful and safe way to put the general plan into effect (checkmate the opponent's King), one most often proceeds in small (positional and) tactical steps, like pinning a piece to one of greater value so that it cannot move out of harm's way without a measure of destruction and desolation ensuing.
Though the middlegame isn't limited to the following considerations, your son should eventually acquaint himself with certain "tactical motifs", amongst which: capturing an unguarded (or "hanging") piece or pawn, destroying a guard, imprisoning a Bishop, forcibly exposing the opponent's King, attacking the weak f7 (if White, f2 if Black) pawn (or square), mating on the back rank, perpetual check, passing a pawn, etc.
Having now presumably clearly understood that tactics are - as Tarrasch and others have pointed out - the most important considerations of the middlegame, I feel compelled to burden your mind with a further fact relating to "sacrifices". The thing about "sacrifices" is that winning combinatorial manoeuvres must often be created or initiated by a "sacrifice". The other thing about "sacrifices" is that, like chess, understanding when one can usefully make a sacrifice is more easily said than done. Ideally, your son will understand this by experimenting with sacrifices and learn from his mistakes and successes.
Well, it seems I have been long-winded again and will not get to the "endgame" exposition at this time...
Let me just close by giving you these middlegame rules of thumb which you may want to pass on to your son: he might usefully experiment with trying to create an objective weakness in his opponents' forces and then attack where he is strong and his opponents weak. Perhaps the simplest way to achieve this, at first, is by creating structural weaknesses in the opponents pawn-chain whereby enemy pawns will be doubled on the same file, isolated from one another by one or more file or backwards and unprotected by another pawn.
Once this is done, a useful general strategy - if his own pawn structure is intact or less compromised than the opponent's - would be to seek to simplify the game by proceeding to force or accept exchanges of pieces and pawns that don't leave him with a clear disadvantage or exposure to some tactic: once the board has been cleared of these pieces and pawns, his opponent's weak structure will become an even greater liability and a winning endgame should, more often than not, ensue, as long as certain endgame principles are understood and followed.
An operational definition of the "endgame phase" as well as certain key rules of thumb will be given in my next instalment.
Hope this is of some use to you and your son, going forward.
