Let's have a look! Edit: I see that Marcus already spoke to a lot of points, so I'll just comment on one or two.
6: I don't think I'd often tenuki an approach on a 3-4 stone, but it's not entirely unreasonable.
- Click Here To Show Diagram Code
[go]$$c Position at move 7
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O , X . . . . , . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . , . . . . . O . . . |
$$ | . . O . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
Okay, here's a good look at the priorities of different approaches. The top right is all stable and fine, so we can call it lowest priority. We see the rest are approaches on a 3-3, 3-4, and 4-4 stone.
From here, Marcus makes a great point. The closer the approach, the more urgent the response. If your opponent touches your stone (diagonals count), it tends to be super-urgent to respond. For this reason, this corner is the most urgent.
- Click Here To Show Diagram Code
[go]$$Wc Position at move 7
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . O , X . . . . , . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . 4 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . 3 . . . . . . . . . . . . . . . . . |
$$ | . . . X 2 . . . . , . . . . . O . . . |
$$ | . . O 1 . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
This is a standard continuation that ends with both groups mostly stable. Here, you have a chance to take sente! Now, however, we're back to a question of priorities. We know that the top right is fine for the moment. That leaves us with an approach on a 3-4 or an approach on a 4-4. In general, balanced corners are lower priorities. By this, I mean that, all other things being even, it is bigger to approach a 3-4 in the first place than it is to approach a 4-4. Similarly, the followup seems to be better, as well. Because the 3-4 is really hemmed in by the approaching stone, let's do something to fight back. We could attach underneath at E17, taking the corner and giving black some thickness. That would look like this:
- Click Here To Show Diagram Code
[go]$$Wc Position at move 7
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . 3 1 2 . . . . . . . . . . . . . |
$$ | . . O , X 4 . . . 6 . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . 5 . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . O . . . . . . . . . . . . . . . . . |
$$ | . . . X X . . . . , . . . . . O . . . |
$$ | . . O O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
However, black would then have influence facing his influence to the south, in addition to making a huge framework on top. That means that black would have a lot of strength for any fighting coming up, not to mention further scary framework potential. Because of that, let's find something else to do. Black's top side looks like it is way too big in this diagram, so how can we break it up? Pincer, and take it for ourselves.
- Click Here To Show Diagram Code
[go]$$Wc Position at move 7
$$ ---------------------------------------
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . 1 . . . . . . . . . . . . |
$$ | . . O , X . . . . , . . . . . X . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . , . . . . . , . . . . . , . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . X . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . X . . . |
$$ | . O . . . . . . . . . . . . . . . . . |
$$ | . . . X X . . . . , . . . . . O . . . |
$$ | . . O O . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ | . . . . . . . . . . . . . . . . . . . |
$$ ---------------------------------------[/go]
After all of this, the issue of corner priority has been solved. Whoever gets to take sente after the pincer fight will jump to deal with the only urgent corner: the bottom right.
I know this whole topic of corner priority has gone much deeper than intended, so if you have any questions, feel free to ask.
I'll just point at a few other moves.
19: This is only worth playing if you are afraid of the cut you are preventing. However, if white plays at E5, his cutting stone dies in a ladder. Can you see the ladder now? If a cut cannot possibly work for your opponent in the forseeable future, you probably don't need to worry about it yet. As an alternative, consider this: you just built a large wall. You urgently want to extend from that wall, and your opponent wants to play halfway to the wall to reduce its power. Stopping to cut let your opponent counter the wall. Instead of defending, if you play O3 or even N3, it comes as a terrifying combination of things: an extension from a large wall, and a double-approach on a weak corner.
I'm going to cut it here, since Marcus did a good job at pointing out big things. Good luck!