The middlegame arises out of the opening and so in the early middlegame the ideas arise out of the opening chosen. For instance, the King's Gambit has the idea of opening the center quickly, allowing white rapid development and an early kingside attack often targeting the naturally weak f7 square. A hypermodern opening like the Modern has the idea of avoiding occupying the center with pawns. Instead it tries to exert control over the center with pieces. At the same time it looks to encourage white to over extend creating the possibility of a counter attack.
So, in the early middlegame the thoughts mostly center around the best moves to make to further these opening related plans. For a tactical opening these plans will be tactical and for more positional openings they will concern the creation of weaknesses in the opponent's position which can be exploited later.
In the late middlegame the emphasis changes. After the middlegame comes the endgame and it becomes more important to start thinking how to transition to a favourable endgame. For example, if you would have a much better endgame then it can make sense to force the exchange of queens, or if that's not possible, give your opponent every opportunity to exchange queens. Conversely, if you would be at a disadvantage in the endgame then try and keep queens on and try and create complex positions where it is easy to go wrong.
Throughout the middlegame, from early to late, it is important to bear in mind that tactics often play a key role in deciding the game and the middlegame is usually the area of the game with the most tactical possibilities. Hence it is important to have a checklist of questions to ask yourself before moving, like:
- What checks do I have?
- What threats do I have?
- What threats does my opponent have?
- What weaknesses does my opponent have that I may be able to exploit?
- What weaknesses do I have that my opponent might be able to exploit?
All of these questions will require some calculation of variations to answer in full. So, two further rules are useful:
- Calculate variations through to the end or even one move beyond because there is almost nothing worse than calculating a variation, playing it and discovering at the end that you missed your opponent's move which turns the position round.
- Sometimes when you see a promising move, A, which you want to follow up with move B, it can be more effective to reverse the moves. Make sure you check both move orders.