Tactics
The trapped piece
A piece with no safe square is as good as won. It does not have to be attacked yet: you attack it once you have taken its squares away. Bishops that have wandered in to take a pawn and queens that have gone hunting are the usual victims, and a rook that has taken on a7 or a2 is the classic.
How to spot it
Look for enemy pieces deep in your position or on the edge of the board, then count the squares they can go to rather than the attackers on them. If the answer is one or two, a pawn move that takes those squares away wins the piece, and it is worth spending a move or even a pawn to do it.
The positions
Easiest first. Every one is from a real game, and the rating under each board is what it costs the players who have tried it on lichess.
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1. Black to play Play it out →Answer
23...Rg6 24.f5 Rxf6
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2. White to play Play it out →Answer
9.Qh5+ g6 10.Nxg6 hxg6 11.Qxh8
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3. Black to play Play it out →Answer
31...Ra5 32.Qxa5 Qxa5
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4. White to play Play it out →Answer
13.Rh1 Qxh1+ 14.Qxh1
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5. Black to play Play it out →Answer
13...Bg4 14.Bxe5 Bxh5
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6. Black to play Play it out →Answer
23...Re8 24.Qxe8+ Bxe8
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7. Black to play Play it out →Answer
26...g5 27.Qxd6 Qxd6
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8. White to play Play it out →Answer
26.Rh3 Qxg1+ 27.Qxg1
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9. Black to play Play it out →Answer
12...Qxf2+ 13.Kd1 Qg2 14.Be1 Qxh1
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10. White to play Play it out →Answer
23.Rb3 Qxb3 24.Qxb3
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11. White to play Play it out →Answer
27.Rh7 Qxh7 28.Qxh7
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12. White to play Play it out →Answer
7.Nxf7 Kxf7 8.Ng5+ Ke8 9.Ne6 Nb6 10.Nxd8
Reading about a pattern is the easy half. Motify finds the positions where you missed one, by running your own lichess and Chess.com games through Stockfish, and puts each of them behind two or three puzzles on the same idea so you meet it having just found it three times. See how Motify works.
Positions from the lichess puzzle database, released under CC0.