Once you learn the single rule of Sudoku and play a few rounds on the easy level, it is time to step up. From the medium level onward, direct logic stops being enough. You scan the board and find no cell with a single possibility. No row is almost complete. You seem stuck. But you are not stuck. You are short on technique.
Before the techniques, one non-negotiable assumption: take notes. In any medium or hard Sudoku, you have to place inside each empty cell the numbers that are still candidates to go there. These small marks, usually in a lighter color, are the clues you will use to apply the techniques below. Without notes, the techniques do not work. In Sudoku BLA, the notes mode is always one tap away, and auto-notes fill the candidates for you when you want a faster start.
Technique 1, naked single. Look at the notes of an empty cell. If only one candidate is left, that number goes there. It sounds obvious, but it is the technique that shows up most often. Every time you place a new number, it gets eliminated from the candidates of all cells in the same row, column and block. That can trigger a cascade of naked singles. Always start your scan with this technique.
Technique 2, hidden single. In a row, column or block, find a number that appears as a candidate in only one cell of that group. Even if that cell has other candidates, this specific number can only go there. It is a number hidden among the others whose place is set by exclusivity.
Technique 3, naked pair. In a row, column or block, find two cells that have exactly the same two candidates, and only those two. Those two numbers will land on those two cells. You do not know yet which goes in which, but you can erase those two numbers from the notes of every other cell in the same group. That clears space for techniques 1 and 2 to start working again.
Technique 4, pointing pair. In a block, look if a candidate number appears only in cells of the same row (or the same column). If so, that number will land in one of those cells of the block. Which means it cannot land in any other cell of the same row (or column) outside that block. You erase that candidate from every other cell on that row (or column) outside the block.
Technique 5, X-wing. Look for a candidate number that appears in exactly two cells of two different rows, with those cells aligned in the same two columns. They form a rectangle. That number will land in two of those four cells, in diagonal. It sounds abstract, but the effect is powerful: you erase that candidate from the other cells of those same two columns. X-wing is the first technique that separates hard from expert.
With those five techniques, you close 95% of the puzzles you will face. The other 5% need more advanced techniques (X-chain, swordfish, coloring techniques), but that is a topic for another post. For now, download Sudoku BLA, switch on notes mode, and practice. The techniques come one at a time, with reps.
