Understanding soundness and motivations in chess puzzles, problems, and studies

What makes chess puzzles, problems, and endgame studies sound – or unsound? Why do some puzzles seem to contain “sub-optimal” moves by the opposing side? Is a problem or a study faulty if its thematic variation isn’t apparently “forced”? These sorts of questions are often raised, and it’s certainly important

Chess Puzzles Brilliant Math & Science Wiki

Are chess tactics and puzzles critical for learning chess?

Glossary of chess - Wikipedia

chess problems by Leonid Kubbel – Daily Chess Musings

1200 Medium Chess Puzzles in Two Moves Printable PDF

An introduction to composed chess problems

WCB PDF, PDF, Chess Openings

How to become a chess composer (make chess puzzles) : r/chess

If a club player only has 2 hours a day for chess improvement, how would you suggest they split it up (puzzles, games, watching lectures, opening theory, etc.)? - Quora

Am I missing something or does the following puzzle not make sense? White to play and mate. : r/chess