Sudoku-YFoundations
When a mistake spreads: trace the reason, not just the duplicate
A visible conflict can be the end of an earlier mistake. Do not erase a whole row just because the problem appears there.
Describe the conflict first
Repeated digits violate the rules, but their absence does not prove a board is correct. An empty candidate set, or a missing digit with nowhere to go in a unit, can also expose a problem. These checks require complete candidates and trustworthy entries.
A small diagnostic example
Row E needs 2 and 9, with E3 and E8 empty. Suppose checking their columns and boxes leaves both with only 2. You cannot enter both. Trace why 9 was removed from each cell; do not arbitrarily restore it in one cell to make the row look complete. If an elimination relied on an assumed value, that dependency is still conditional.
Walk back through dependencies
Start with the latest related change. Was it a given, a proved placement or a trial? Recheck conclusions built from that value. Undo can recover a previous state, but does not identify the faulty argument for you. A correct-looking later entry may need reconsideration if its premise was wrong.
Keep different kinds of information separate
Use small notes for candidates and the assumption tool for trials. Do not remove a candidate merely because it feels unlikely. Rebuild incomplete notes from placed digits before using them as evidence for another technique.
Try before looking
Try before revealing
A row has no placed duplicates, but two empty cells each allow only 5. Is it safe?
Show explanation
No. With complete, correct candidate lists, both cells would require the same digit. The contradiction exists before either 5 is entered.
References and further reading
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