Outcome
You can reject an illegal placement by checking the cell's row, column, and box — and you know that a legal candidate is not automatically the answer.
Intro copy
Every placement gets the same three checks:
row → column → box
If the digit already appears in any one of them, it cannot go in the cell.
Step 1 — Row conflict
Prompt:
Can 6 go in the highlighted cell?
State requirement:
A solved 6 is visible elsewhere in the target row. Column and box should not introduce a second reason in this first example.
Correct feedback:
No. The row already contains 6.
Incorrect feedback:
Follow the highlighted row. There is already a 6 in it.
Step 2 — Column conflict
Use a new controlled example with only the column providing the obvious conflict.
Step 3 — Box conflict
Use a new controlled example with only the box providing the obvious conflict.
Step 4 — A legal candidate is not always solved
Prompt:
The digit 4 does not appear in this cell's row, column, or box. What does that tell us?
Correct answer:
4 is still possible here.
Incorrect option to teach against:
This cell must be 4.
Correct feedback:
Good. The rules tell us 4 is a candidate. We still need more logic before placing it if other candidates remain.
Practice
Five placement checks mixing row, column, box, and legal-but-not-forced examples.
Avoid examples with accidental multiple conflicts when the lesson is testing one specific rule.
Completion
4/5 correct. Incorrect answers can be retried after feedback.
Next
Your First Moves — use the rules to solve cells instead of only ruling digits out.
Reference
Read Sudoku Rules.