Lesson 1 - The Basic 3-3 invasion
There are plenty of openings (joseki) that you should learn to improve quickly as a beginner. However, it is undeniable that the most opening that is most frustrating not to know is the one shown below.
Replay Applet: ZGo
With move 1, Black begins a loose corner position by playing the 4-4 (hoshi) point. This point has the potential for great outside influence, but has a major flaw in terms of securing the corner. White's move 2 exploits this flaw at the 3-3 point. Black then has a choice of two positions for move 3 - Q17 and R16. If black wants the topside (perhaps already having a stone in the upper left corner), then the play in the example, Q17, makes sense. If, on the other hand, black has the lower right corner, or wants to develop the right side, then R16 is the correct play. The moves that follow are the most common sequence. Black's R13 prevents white from taking that point and breaking out of the tight corner (and also continues black's wall at minimal cost). Black's O17 seems slow, but black needs to come away with a strong wall to make this opening profitable, and the move protects against a cut that could undermine that wall.
This sequence, standing on an open board, benefits black. White only gains 8-10 points, while black's wall gives exactly the influence that the 4-4 play was designed for. However, white has the next move after the sequence (sente - the initiative), and sente is all important in openings. White's next move can reduce the influence of black's wall, but ideally this opening should be played with stones already played in each corner, and some development on the sides.
White would prefer to invade when black has low positions on the top and right. For instance, if black had stones at K17 and R10, then the wall would have low-placed support, and therefore be worth less than if black had stones at K16 and Q10 instead. The K17-Q16-R10 formation is known as the low double wing, while K16-Q16-Q10 is an example of the regular, high double wing. Building a low double wing formation is equivalent to begging your opponent to invade at the 3-3 point, because the benefits to that invasion would outweigh the costs.
This sequence will work as long as there are no black stones closer to the 4-4 than M17 or R12. There are other 3-3 sequences that work with tighter corners, and we cover those in later lessons.