By the way, if a tsumego occurs in an actual game, not doing anything locally is often (not always) the best option for the second player. The one you recently posted is not an exception. The reason is white has different ways to use the aji (P15 is one way and M16, for example, is one of others) and wants to keep all those possibilities until they have to make a choice. Another factor white needs to consider is whether black will use a move to fix the aji. In this position probably not since it’s slow. Or in other words, if black does fix the aji white is still happy due to black’s inefficiency
.
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In fact it’s good advice for DDK (at least) that when you are about to be gote, giving the choice to your opponent to play where he wants, it’s a good time for you too to change mind if you could get more sonewhere else. Consider in that case that you are going to play 2 moves if your opponent still persist to play where you are (like to make his group alive).
The right time to Tenuki (changing place to play) is still difficult. Consider it when it’s just a matter of cash, but be very cautious when it’s a matter of architecture (push, cut, connect…). Cash which make your stones connected are of highest value.
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My gues is that can train you for go too (if not confusing).
Snapback, connect&die and squeeze are 3 of the most important techniques to think about while trying to get creative about a position. If you start out with connect&die in mind, it’ll be easier to detect it.
Also, you can think about liberties. When you throw in, white will have to take for sure - that is just one step reading. When they take, seeing that they will be taking their own liberty (cuz they’ll be touching your stone, t5 - t4) might be worth noting. They will also be making an empty triangle, which is usually inefficient shape in terms of liberties. Seeing the throw-in results in an empty triangle that takes away its own liberty in the very next move could make it easier to settle on the throw-in when browsing the position initially.
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So, my thinking was B S5, to which W responds with S4 and says “wrong”. But B T4 would be my next move. Why is it wrong? AI suggests S4, which OK, but don’t I kill everything anyway?
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With black S5 it’s a ko fight. Black S4 kills white’s corner stones unconditionally.
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Hm, it’s a mistake I keep repeating it seems.
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If it’s any consolation, it took me a little while to work this one out for the same reason.
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Whenever you see a ko square on the board after the last move in the variation, it is likely that the life/death result will be decided by that ko:
(That square means that black is not allowed to recapture there immediately, but could do so after making a ko threat)
It can still be tricky to understand why a particular ko is right or wrong, but the square is an easy way to recognize that a ko is involved somehow.
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I’ve never noticed that ko square before I don’t think. Thanks 
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https://www.101weiqi.com/spec/dajie/
Weiqi101 has a ko tsumego collection with over 2300 problems; could help with recognizing them in the future.
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Ko recognition and assessment can become better simply by playing them in your games. Just a matter of cumulated experience. It’s something not very natural at first but if your games miss kos you can even create some, no joke! Think on it when attacked, and you feel to move too slowly to escape.
Enjoy kos as much as cutting or killing or pressing…
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Oh that’s a lot. 
But tbh I need the practice. 
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I’m guilty of the opposite, actually. 
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I’m not sure I understand the goal in this…
I can’t kill, or get two eyes.
I saw the solution, but I don’t see how the ending position works for me.
Or maybe I’m rusty.
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Is the idea
to
either connect out with the stones in the corner so they live and steal the points, or to capture the four white stones?
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Summary
I don’t think those 4 white stones die in the suggested solution, so I don’t get what I’m supposed to aim for. 
In terms of what to aim for I can give some ideas of expectations, but there’s spoilers below. I know you know the solution, but there’s maybe one extra variation including (assuming I’m reading it right )
Connect out the stones.
The idea is to exploit the shape and shortage of liberties. I’ll assume I’ve gotten the first move correct a and b look like key moves but the order seems to be important.
So with 1 it aims to connect the black stones to the outside on the first line. If white knows what happens if they resist, or they just capture the stone instinctively black can connect on the first line. White can’t play the X because it’s self Atari, and being close to corner means it takes white an extra move to start filling liberties from the far side (they can’t play in the corner immediately.)
details within details
Even if white tries a throw in type trick I think they’re just a move behind
Without the throwing might be slightly better and black might give up the T18 stone at least.
Capture the stones if they resist
If white tries to stop it Black can save the stone in Atari, and white doesn’t have enough liberties to Atari/capture because the corner is so close. T12 will be Atari for black next move.
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Oh I need to look at this closer after work…
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Take this problem like an endgame problem, black to finish the boundary.
solved
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