user26139: I am Black and my opponent is White. Here the computer recommends Rxd4, which I have done (the rook captured bishop here).
[FEN "4r1k1/1pp2pp1/p6p/4q3/P2rPn2/2P1QP1P/1P4P1/R4RK1 w - - 0 1"]

It's obvious then that Qxd4 is a blunder, because then I play Ne2 to fork the king and queen and the game is over. I would expect the computer to play cxd4 here to go up material, but the computer plays Rfe1.
Why not cxd4 here? What future sequence am I not seeing?

user-1: Stockfish, at depth 20, evaluates Rfe1 with a score of -8.5, while cxd4 has one of -2.6. With so large a difference, the explanation is that
you were playing against a weak computer. There is usually
a gradient (e.g. from 1 to 10) of difficulty to choose from
when playing against a computer, so you have probably chosen something
far from 10. Chess.com (guessing from the screenshot) also has
adaptative opponents that will grow stronger/weaker according to your
performance.
For what it's worth, shortly after cxd4 White gives up a rook and a pawn
for Black's knight and has an exposed king and piteous pawn structure —so Black wins anyway—, as follows.
4r1k1/1pp2pp1/p6p/4q3/P2rPn2/2P1QP1P/1P4P1/R4RK1 w - - 0 6

1. cxd4 Qg5 2. Kf2 Qxg2+ 3. Ke1 Qxb2 {Double threat: Qxa1+ and Ng2+ (a royal fork)} 4. Qxf4 Qxa1+ 5. Ke2 Qxd4


26139: <p>I am Black and my opponent is White. Here the computer recommends Rxd4, which I have done (the rook captured bishop here).</p>
<pre><code>[FEN &quot;4r1k1/1pp2pp1/p6p/4q3/P2rPn2/2P1QP1P/1P4P1/R4RK1 w - - 0 1&quot;]
</code></pre>
<p>It's obvious then that Qxd4 is a blunder, because then I play Ne2 to fork the king and queen and the game is over. I would expect the computer to play cxd4 here to go up material, but the computer plays Rfe1.</p>
<p>Why not cxd4 here? What future sequence am I not seeing?</p>
 -1: <p>Stockfish, at depth 20, evaluates Rfe1 with a score of -8.5, while cxd4 has one of -2.6. With so large a difference, the explanation is that
you were playing against a <em>weak</em> computer. There is usually
a gradient (e.g. from 1 to 10) of difficulty to choose from
when playing against a computer, so you have probably chosen something
far from 10. Chess.com (guessing from the screenshot) also has
adaptative opponents that will grow stronger/weaker according to your
performance.</p>
<p>For what it's worth, shortly after cxd4 White gives up a rook and a pawn
for Black's knight and has an exposed king and piteous pawn structure —so Black wins anyway—, as follows.</p>
<pre><code>4r1k1/1pp2pp1/p6p/4q3/P2rPn2/2P1QP1P/1P4P1/R4RK1 w - - 0 6

1. cxd4 Qg5 2. Kf2 Qxg2+ 3. Ke1 Qxb2 {Double threat: Qxa1+ and Ng2+ (a royal fork)} 4. Qxf4 Qxa1+ 5. Ke2 Qxd4
</code></pre>
