Beginner sudoku mistakes: How to avoid them
Avoid common beginner sudoku mistakes with worked examples. Check candidates, distinguish singles and pairs, and recover from an incorrect entry.
1. Treating a possible entry as a proven answer
A common beginner sudoku mistake is entering a digit because it does not visibly repeat. A 4 fitting in a cell does not prove it is the answer; the same cell might also allow 7. A placement needs a reason that excludes the alternatives or shows the digit has no other place in a unit. Before confirming a digit, finish the sentence “this cell is 4 because…”. If your only reason is that you cannot see another 4 nearby, you have a candidate rather than a proven answer. Keep it as a pencil mark and check the whole row, column and box.
2. Guessing when two candidates remain
A list of {2, 8} does not tell you to choose whichever digit feels more likely. The number of 2s already visible elsewhere in the grid does not settle this cell either. Both options remain open until another deduction supplies a reason. For example, if a proven move later places 8 in the cell’s column, you can exclude 8 and enter 2. Until then, inspect another unit or try a different technique. These guides focus on explained deductions. If you explore a hypothesis while playing privately, keep it distinct from proven entries and retain a way to undo it.
3. Reading a missing pencil mark as an exclusion
Partial notes are reminders rather than complete candidate lists. Suppose you wrote {3, 6} in a cell but never checked whether 9 also fits. Removing 3 after another move does not yet prove the cell is 6. First check the possibility you omitted. This mistake can create a false naked single and spread incorrect entries across several units. Decide what your notes represent before treating them as evidence. To confirm a sole candidate, rebuild its possibilities from the row, column and box, while retaining valid eliminations you previously established through other techniques. An incomplete list cannot prove its missing digits are impossible.
4. Looking only for cells with one pencil mark
Sometimes the next move is a digit with just one location, even though that cell contains several marks. Consider a local row situation with three empty cells and complete lists {1, 5}, {1, 5, 8} and {1, 5}. Only the middle cell allows 8, so it must be 8. This is a hidden single. The remaining cells are still undecided between 1 and 5. Inspect columns and boxes with the same question: Where can each missing digit go? This exercise describes only one unit and assumes its candidate lists have already been checked against the rest of the grid.
5. Extending a pair beyond its shared unit
Two cells containing only {3, 7} form a naked pair when their complete lists match and they share a row, column or box. You can then remove 3 and 7 from other cells in that shared unit. If r4c1 and r4c8 form the pair, the eliminations apply to row 4. They do not let you remove 3 from the whole of column 1 or from every box touched by the pair. There is no naked pair if the second list is {3, 7, 9}. Check the two cells first, their shared unit second, and each intended elimination last.
6. Changing several entries to fix a contradiction
Two identical digits in a unit are a reason to review your earlier steps. Erasing several answers at random usually makes their source harder to find. Return to the last placement whose reason you can explain, then inspect later decisions. An incorrect note, a mistyped entry and a faulty deduction need different corrections. If your game offers undo, use it to revisit those moves in order. After withdrawing an answer, review which notes depended on it before continuing. Restoring a lost candidate may also be necessary to make the notes consistent with the grid again. Avoid building further deductions on an unresolved contradiction.
7. Try spotting the unsupported move
Three cells remain in a row, with complete lists r6c2 {2, 9}, r6c5 {2, 9} and r6c7 {2, 4, 9}. One player enters 2 in r6c2 because it comes first in the list. Another removes 2 and 9 from r6c7 and enters 4. Only the second move is proved. The first two cells reserve 2 and 9 without fixing their order, so the third must contain 4. Update the units through r6c7 and continue. If r6c5 also allowed 5, the pair would disappear and this argument would no longer justify the 4 either.
8. Use a short check before moving on
Before a placement, check the selected cell and state your reason. Afterwards, update its row, column and box. When a technique removes candidates, confirm its conditions and keep every elimination within its proper scope. If you cannot find a move, review a lesson, choose an easier grid or use a practice hint and reconstruct its explanation. Learning does not require finishing every puzzle unaided. What helps is recognising what you know, what remains undecided and what information would let you progress. Choose one mistake from this guide to watch for in your next game, without turning every move into a test.
Now try it on the grid.
Practise at your own pace and look for a chance to explain the next deduction.
Practise with a Sudoku