Do you have to guess in Sudoku?
No, you do not have to guess in a properly constructed Sudoku. Every placement can be justified by the clues and previous deductions, although a very hard puzzle may require a long chain of candidate reasoning before the next digit is forced.
Guessing is not the same as testing logic
A blind guess places one of several candidates and hopes it works. A forcing chain temporarily follows a candidate's consequences to prove that it creates a contradiction or that every possible branch reaches the same result. The second method is still logic because the conclusion accounts for every case.
What to do when you feel forced to guess
- Refresh all candidates; stale pencil marks are a common cause of dead ends.
- Scan one digit across all rows, columns, and boxes instead of staring at one cell.
- Look for pairs, triples, and pointing candidates before advanced patterns.
- Use one hint from the Sudoku solver, then work backward to understand why the answer was forced.