Contact
No contact form, because a form needs a server and a database. One address, read by a person.
hello@sudokuexplained.com
There is no form on this site, on purpose: a form would mean collecting your address on a server, and this site does not have one.
What is worth writing about
An explanation that is wrong. The most valuable kind of report. Include the difficulty, roughly when you played it, and which step in the hint chain was wrong — a screenshot of the derivation is ideal. Every explanation is generated from the board, so a wrong one is a real bug and can usually be reproduced.
A board that does not match its article. The figures in the technique guide show only the relevant candidates, which can look odd at first glance; but if a coordinate in the text contradicts a number in the picture, that is worth reporting.
An ad that misbehaves. Ads that cover the board, make noise, or redirect you somewhere unpleasant should be reported to the ad network as well — but telling me helps, because it is possible to block specific categories.
A puzzle with more than one solution. Rare, but it happens if a bug slips through. Send the position and it will be replaced.
What is not
Puzzle requests, book recommendations and general sudoku questions all get read, but they may not get an answer. The articles and the technique guide cover most of what people ask.
Response time
This is a one-person project, so replies are not instant. Genuine bugs are usually fixed within a week; everything else depends on how much time the puzzles are leaving.