By difficulty
Long calculation
Puzzles whose solution runs five moves or longer. The difficulty is not the ideas, which are usually ordinary, it is holding the position in your head at the end of the line and still being able to see what is on which square.
How to spot it
Calculate one line to the end before starting another, and say the moves to yourself in notation rather than picturing them: the words are what stop the board in your head drifting. When a line fails, go back to the branch rather than to the beginning.
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
15.Qxe6+ Be7 16.Qf7+ Kd8 17.Ne6+ Kc8 18.Qe8+ Bd8 19.Qxd8#
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2. Black to play Play it out →Answer
19...Rh1+ 20.Kxh1 Qh4+ 21.Kg1 Qh2+ 22.Kf1 Qh1+ 23.Ke2 Qxg2+ 24.Ke1 Qf2#
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3. Black to play Play it out →Answer
54...Bxe8 55.Kxe8 Kf5 56.Ke7 Kg4 57.Bd6 Kf3
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4. Black to play Play it out →Answer
27...Rxf2+ 28.Kxf2 Qxg3+ 29.Ke2 Qg2+ 30.Kd3 Rg3+ 31.Kc4 Qxe4
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5. Black to play Play it out →Answer
26...Qxf2+ 27.Rxf2 Rb1+ 28.Bd1 Rxd1+ 29.Rf1 Bd4+ 30.Kh1 Rdxf1#
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6. Black to play Play it out →Answer
38...Nc2+ 39.Ka4 Nb2+ 40.Kb5 a6+ 41.Kxa6 Ra8+ 42.Kb5 Na3#
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7. Black to play Play it out →Answer
16...Rc2+ 17.Kf1 Qc4+ 18.Kg1 Rc1+ 19.Rxc1 Qxc1#
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8. Black to play Play it out →Answer
25...Nxd6 26.Qxd6 Rxf1+ 27.Kxf1 Qf7+ 28.Kg1 Qxc4
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9. White to play Play it out →Answer
46.g4+ Kh6 47.g5+ Kh5 48.Bd5 Qxh4+ 49.Rxh4+
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10. Black to play Play it out →Answer
32...Qg2+ 33.Rxg2 Nf3+ 34.Kh1 Re1+ 35.Nf1 Rxf1+ 36.Rg1 Rxg1#
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11. White to play Play it out →Answer
37.Qxg7+ Rxg7 38.Nf6+ Kh8 39.Rxd8+ Rg8 40.Rxg8#
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12. Black to play Play it out →Answer
33...Qc1+ 34.Rd1 Qxe3+ 35.Kh1 Qe1+ 36.Rxe1 Rxe1#
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.