I was reviewing a game and the AI said that move 71 was -82. I don’t understand why. I was trying to block white so I could potentially capture the whole group. (If the group was already alive, please do explain that too.)
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The blue move (which is the best according to the AI analysis, and it’s correct) explains why: if you block at e9 then white can play at c2 and split your corner in two, killing it, and thus living. Instead if you play c2 then now your lower left corner is safe, and white is dead because white playing at e9 doesn’t help white make any more eyes, just destroys a few points of potentially black territory up there, but if the group poking into your territory only has one eye and is dead then it’s still your territory.
The general lessons to learn here are:
- watch our for and take care of your weaknesses before trying to attack the opponent: your corner needs to defend at c2 to live
- when killing groups you don’t need to take all their liberties (like e9 here), you need to stop them making 2 eyes. If their move does not threaten to make eyes, you don’t need to respond to that threat to encroach in your territory.
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Oh, I get it now. Thanks for clearing that up.
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You see? Making progress 
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Perhaps you can draw a couple of more general conclusions from this situation:
- Pushing into a large, loosely defended space is usually more effective than pushing into a small, tightly defended space. That means White probably wants to push to C2 rather than E9 — so that is where you should block.
- Play where you are weak (but not hopelessly), not where you are strong. You are very strong around E9 but in danger around C2.
- Do not just think about your own moves and how you can attack your opponent; think how they might answer your moves.
- It looks a bit as though you did not think how White could answer you, but perhaps you just thought they would have to respond locally(!). Or maybe you did not have or take enough time to think about it.
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To enhance a bit what @Uberdude pointed out, White is dead even from this move:
All you need to win is play c2. White has only one eye at A and cannot make a second eye in the square marked space and the triangle marked point is not an eye, since it will belong to a White group that can be reduced to one liberty.
For example:
Black can play at A and capture all those stones, so this is not an eye.
I’d like to add that even if White could make two eyes at that point, all you need to win is c2. That would have made the score:
White: 2+komi.
Black: around 20 points
The final goal of the game is to get more territory, not to kill the opposing groups. 
If you can get points by killing stones, then that’s good, but it is the points that matter.
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