Naked and hidden singles
The two moves that solve most Easy and Medium puzzles.
A naked single is a cell where only one number is still possible. Look along the cell’s row, its column and its box: if eight different numbers already appear among them, the ninth is the answer.
A hidden single works the other way round. Pick a number, say 7, and one box (or row, or column). If 7 can only go in one cell of that box because every other cell is blocked by a 7 in the same row or column, then that cell is 7, even if it could hold other numbers too.
Good habit: scan number by number. Start with the number that already appears most often in the grid, check each box that is still missing it, and place every hidden single you find. Then move to the next number.
Practise it
The Hint button on every puzzle explains the next step in the same plain language, so you can learn as you play. Play a Diagonal Sudoku puzzle.