A hidden single looks nothing like a naked single at first glance — the cell itself might still show three or four pencil marks. What’s “hidden” is that, somewhere in that cell’s row, column, or box, one digit has nowhere else to go, even though the cell doesn’t look resolved on its own.
The difference that matters
A naked single is a property of one cell: its own candidates narrowed to one. A hidden single is a property of a unit — a row, column, or box — where a digit’s remaining candidate cells narrowed to one, even if that cell still has other candidates sitting alongside it.
That distinction is why hidden singles get missed. If you only ever look at how many pencil marks are in a cell, you’ll walk right past a hidden single sitting in a cell with three or four marks in it, because nothing about the cell itself looks “nearly solved.”
Worked example
In row 7, 3 can only go in r7c8.
Before you toggle to “after,” try to find the digit yourself: pick a box or row on the board and check each digit 1-9 against it — does it already appear? If not, how many of that unit’s empty cells could still take it? When the answer is one, that cell is a hidden single for that digit, regardless of anything else written in its corner.
How to find one quickly
Work unit by unit instead of cell by cell. For a box you suspect is close to done, go through digits 1-9 in order and ask “which empty cells in this box could still take this digit?” The moment you hit a digit with only one possible cell left, you’ve found it — even if that cell has other pencil marks sitting next to it that you haven’t eliminated yet.
Related techniques
Hidden singles and naked singles together are the only two techniques an Easy puzzle on this site ever needs. When a board stops yielding either one, locked candidates is usually the next thing to check — it eliminates candidates rather than placing digits, which is a different kind of move worth getting used to.
See how to use pencil marks if tracking candidates by hand is what’s making hidden singles hard to spot.