Five habits that keep you slow at Sudoku

Improving at Sudoku is usually framed as learning more techniques. That is half of it. The other half is dropping the habits that stop the techniques you already know from working — and those are much easier to fix, because you do not have to learn anything to stop doing something.

Here are the five that cost people the most, roughly in order of how much time they waste.

1. Guessing when you get stuck

This is the expensive one, and it is expensive in a way that is easy to miss.

A guess that turns out wrong does not cost you one cell. It costs you every cell you filled in afterwards, plus the time spent discovering the contradiction, plus the far worse case where the contradiction shows up twenty moves later and you can no longer tell which of your entries were deduced and which were assumed.

A properly made Sudoku never requires a guess. If you are stuck, the information is there and you have not found it. That is a completely different problem from "this puzzle is impossible", and it has a completely different fix: look for a different kind of move, not a different cell.

2. Not writing candidates down

Plenty of people solve Easy and Medium puzzles entirely in their head and conclude that pencil marks are for beginners. Then they hit Hard and stall.

The reason is structural. Easy puzzles are solved by placing digits — you find a cell where only one number fits and you write it. Harder puzzles are solved by eliminating them: you prove that two cells between them must hold a 3 and a 7, which tells you nothing about those two cells but removes 3 and 7 from every other cell in the row.

An elimination you cannot see is an elimination you cannot use. Candidates are how you see them. This is not a crutch — above a certain difficulty it is the whole game.

3. Scanning at random

The most common way to search a grid is to look at whichever empty cell your eye lands on and ask "what could go here?" — which is a question with about nine answers and no structure.

The systematic version asks the opposite question. Pick a digit, say 4. Find every 4 already on the grid. Every row, column and box containing one is now off limits for a 4. Look at what is left. Very often a box has exactly one cell where a 4 can still go.

Then do 5. Then 6. It feels slower for the first thirty seconds and is dramatically faster after that, because you are answering one question about the whole grid instead of nine questions about one cell. There is a name for the systematic version of this and a guide that walks through it.

4. Forgetting that boxes are constraints too

Rows and columns are visually obvious — they are long lines across the page. The 3×3 boxes are just as binding and much easier to overlook, especially near the centre of the grid where box edges do not line up with anything your eye is following.

A large share of "I checked everything and nothing works" moments turn out to be a box constraint that was never checked. When you are stuck, checking the three boxes touching your problem cell is a five-second habit that pays constantly.

5. Restarting instead of finding the error

You reach a contradiction — two 8s in a row, or a cell with no candidates left. The instinctive response is to clear everything and begin again.

That throws away the most useful information you will get all puzzle. A contradiction tells you that one specific earlier entry was wrong, and finding *which* one teaches you the exact deduction you got wrong. Starting over teaches you nothing and, in the usual case, walks you straight back into the same mistake at the same place.

Work backwards from the contradiction instead. Which cells depend on it? Which of those was the first you were not completely certain about? That cell is almost always the culprit.

What to do with this

Pick one. These are habits, and habits change one at a time — trying to fix all five in one puzzle mostly produces a self-conscious mess.

If you want something to practise on, the daily challenge publishes one graded puzzle a day and a complete worked solution the following morning, so you can compare your route through the grid with a solver's. And when you hit a wall that is genuinely a missing technique rather than a habit, the guides go through them one at a time, in the order the puzzles will actually demand them.