Sudoku is pattern recognition. The same handful of logical patterns appear in every puzzle. Once you know them, "stuck" becomes "I haven't applied this pattern yet."
One candidate remains in a cell. Every other number exists in its row, column, or box. Place it immediately. These should be reflex — check for them constantly.
A number has only one valid cell within a row, column, or box — but that cell may have other candidates too. The number is "hidden" among them.
Two cells in the same group contain exactly the same two candidates. Those numbers must go in those two cells — in some order. Eliminate both from every other cell in the group.
Two numbers appear as candidates in exactly two cells within a group — and nowhere else in that group. Those two cells must hold those two numbers. Remove all other candidates from those cells.
Within a box, a number's candidates are all in the same row (or column). That number cannot appear elsewhere in that row (or column) outside the box.
The reverse: within a row or column, a number's candidates are confined to one box. Eliminate that number from other cells in the same box outside the row/column.
A number appears as a candidate in exactly two cells in row A and exactly two cells in row B, with both pairs in the same two columns. That number can be eliminated from all other cells in those two columns.
Apply these in order of complexity. Most Easy puzzles only need patterns 1-2. Hard puzzles need all seven.
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