Stones on the board are easier to defend in groups; connecting a group of stones makes it harder to capture, since the opponent would need to cover all the liberties of the group, capturing the group completely, rather than capturing single stones. A good tactic to employ, then, is to attempt to divide the opponent into separate groups, while keeping one's own stones connected. (Note that when Black starts with a large number of handicap moves, his stones are mainly useful for this purpose; the White player's stones are threatened immediately with separation, while Black has many potential connections to begin with.)
A key concept in the tactics of Go, though not part of the rules, is the classification of groups of stones into alive, dead or unsettled.
At the end of the game, groups that cannot avoid being captured during normal play are removed as captures. These stones are dead. Groups can reach this state much earlier during play; a group of stones can quickly run out of options and further play to save it is fruitless. Similarly, further play to kill such a group is often of no benefit (unless required to gain liberties for an own group), since if it remains on the board at the end of the game it is captured anyway. Thus groups can be considered "dead as they stand", or just dead, by both sides during the course of the game.
Groups enclosing an area completely can be harder to kill. Normally, when a play causes an area completely enclosed by the opponent to become filled, the group filling the area is captured since it has no remaining liberties (such a play is called "suicide", for obvious reasons). Only if the last play inside the area would kill the enclosing group, thus freeing one or more liberties for the group that filled the space, can the play be considered. This can only be achieved if the liberties on the outside of the enclosing group have been covered first. Thus, enclosing an area of one or more liberties (called an eye) can make the group harder to kill, since the opponent must cover all of its external liberties before covering the final, internal liberty.
From this, it is possible to create groups that cannot be killed at all. If a group encloses two or more separate areas (two or more eyes), the opponent cannot simultaneously fill both of them with a single play, and thus can never play on the last liberty of the group. Such a group, or a group that cannot be prevented from forming such an enclosure, is called alive.
Groups which are not definitely alive nor definitely dead are sometimes called unsettled groups. Much of the tactical fighting in Go focuses on making one's own groups live, by ensuring they can make two eyes, and on making the opponent's groups die, by denying them two eyes.
Determining ahead of time whether a group is currently alive, dead, or unsettled, requires the ability to extrapolate from the current position and imagine possible plays by both sides, the best responses to those plays, the best responses to those responses, and so on. This is called reading ahead, or just reading, and it is a skill that grows with experience. Many players study books of life and death problems to increase their skill at reading more and more complicated positions.
A position that cannot be attacked, that is, one which can easily get two eyes and for which there are not many moves that have to be answered, is called 'thick'. Thick positions are important as they radiate influence across the board. An error that is often made by weaker players is to make territory in front of their thick position; this is an inefficient way of scoring points. The correct way to use thickness is to have it help in the attack of a weak group of the opponent, or in the saving of a weak group of your own.
A light group is also one that is hard to attack, but for a different reason. If a group has a large number of options, often including the sacrifice of part of it, it is called light. Because it is usually impossible to take away all or almost all options, attacking such a group is very hard for the opponent, attacking such a group will bring little advantage. A weak group which cannot be sacrificed at will is called heavy.
A large part of the middle game of a game of Go is usually spent by one player attacking the other player's weak group(s). What is important to remember is that in most cases the goal of an attack is not to kill the attacked group, but to gain territory or influence. The attack is more or less used to restrict the opponent's options and make it impossible for him to make territory or influence himself.
See Go concepts
When a ko occurs, the player whose stone was captured to start the ko cannot immediately recapture; he must play elsewhere to progress the game so that a repeating position does not occur. However, this allows the player who started the ko to deny the opponent the chance to recapture the stone (usually by connecting that stone back to an own group, or by capturing a further stone to free more liberties). Thus, the player finding a move elsewhere would prefer to play such that the originator of the ko finds it more valuable to respond to this new play, than to ignore it and complete the ko. Such a move, threatening to gain advantage if it is ignored to complete the ko, is termed a ko threat.
Once the ko threat is made, the player originating the ko has to choose between responding to the threat, or completing the ko. If she responds to the threat, the player who made the threat can then return and recapture the stone that started the ko. The situation then becomes reversed; the player whose stone was recaptured wants to find a ko threat that allows her to recapture again. Such a sequence of moves -- starting a ko, followed by a ko threat, followed by recapturing the ko, followed by another ko threat and so on -- is termed a ko fight.
Eventually one player will win the ko by ignoring the latest ko threat to complete the ko. The player who made the last threat therefore gets to follow up on it, in effect getting two moves in another part of the board while the opponent completes the ko. It is thus worthwhile choosing ko threats that will give a definite advantage if ignored.
Before deciding to start a ko, it is worthwhile evaluating what threats are available to both players, so that one can decide which side is likely to win the ko fight. Many of the playing skills come together in ko fighting (evaluating the value of moves; reading ahead to find likely moves of the opponent and best responses; choosing the best order of moves), and it is a topic of much discussion among players. This also causes many beginners to be fearful of fighting a ko, since they are not confident of their ability to evaluate threats.
'Sente' and 'gote' are terms that are much used in Go theory. The player who has sente is the one whose move it is once a local continuation has been played out. The player who does not have sente has gote. A sequence is said to have been played in sente if the player who started it has sente afterward. Having or losing sente is important, as taking gote unnecessarily is equivalent to passing.
The first moves are usually played on or near the 4-4 star points in the corners, because in those places it is easiest to make territory. After that, standard sequences (joseki) are used to divide corners, and extensions along the side are made. Usually, the center area is kept empty the longest. Plays are usually on the third or fourth line - the second makes too little territory, while the fifth is too easily undermined by a play on the third. A play on the fourth line is directed more towards influence to the center, a play on the third line more towards making territory along the side.
In the endgame, if the game is close, moves that are small are still worth some points, some more than others. One must chose which of these moves is more urgent to play based not only on the points it may gain, but on whether that move is sente. Yose refers to a specific kind of endgame play, which yields a reduction for your opponent. Generally, in the endgame, all the terrritory is staked out - there is no more to be gained. However, there are still points to be made, as well as possible ways of reducing small amounts of your opponents territory. A simple example would be a move that is dame (neutral point for you) but when its filled in, its sente, requiring white to fill a stone in his territory to answer. We say this is 'a one point reduction, with sente.'
To continue, see Go concepts
Also Go theoryConnection and Separation
Life and Death
Reading
High and Low
Thickness and Lightness
Attack and Defense
Territory and Influence
Ko Fighting
Sente and Gote
Opening (fuseki)
Endgame (yose)