Tactics
The pin
A pin is a piece that cannot move without exposing something behind it. Only the three long-range pieces can pin, because a pin is a line, and the piece at the far end of that line is what makes the one in front of it a target rather than a defender.
How to spot it
Look down every rank, file and diagonal your bishops, rooks and queen sit on, and ask what is lined up behind the first enemy piece you meet. Then remember the second half, which is the half people forget: a pinned piece is not just stuck, it has stopped defending everything it was defending.
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. White to play Play it out →Answer
13.Bb5 O-O 14.Bxd7
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2. White to play Play it out →Answer
19.Rd8#
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3. Black to play Play it out →Answer
24...Rd2+ 25.Kg3 Bxe3
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4. White to play Play it out →Answer
57.Rf5 Rxf5 58.c8=Q
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5. Black to play Play it out →Answer
49...Rxg5+ 50.Kf3 Rxg2
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6. Black to play Play it out →Answer
32...Qxh3+ 33.Kg1 Qxg2#
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7. Black to play Play it out →Answer
56...Qe5#
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8. Black to play Play it out →Answer
33...Nf4+ 34.exf4 Rxc6
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9. Black to play Play it out →Answer
30...Nf4+ 31.Kf1 Qxg2#
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10. Black to play Play it out →Answer
9...Rxc1 10.Qxc1 Nd3+ 11.Ke2 Nxc1+
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11. White to play Play it out →Answer
26.Qa8+ Bxa8 27.Rxa8+ Nb8 28.Ba4+ Nc6 29.Bxc6#
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12. White to play Play it out →Answer
26.Qxf8+ Kxf8 27.Ne6+ Kg8 28.Rd8+ Kh7 29.Rxf7+ Kh6 30.Rh8#
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.