Medium sudoku – today's 4 free puzzles

Why easy feels fine and medium suddenly stops you

Almost every solver hits this wall. Easy grids are built so that a forced cell is always visible somewhere; medium grids deliberately include positions where scanning alone finds nothing. So the wall is not a sign that you lack ability, it is a sign that it is time to pick up one more tool. That tool is the candidate note.

It's fine to pencil in everything at first

Candidate notes are the small digits you write into a cell to record what could still go there. Experienced solvers annotate selectively, but while you are learning there is nothing wrong with filling in candidates for every empty cell. Yes, it takes a few minutes — and those minutes are not wasted, because the act of writing them is how you learn the board: "this row is missing 1 and 4", "this box is waiting on a 7". Then, every time you confirm a digit, erase it from the notes in that row, that column and that box. Doing that erasure carefully is roughly ninety per cent of solving at this level. Later you will drift naturally into shortcuts: only noting cells with three or fewer candidates, or starting notes only in the second half of a puzzle. There is no need to imitate an expert on day one.

Your way in is the two-candidate cell

Once the notes are down, look for cells with exactly two candidates. Even when such a cell can't be resolved directly, a single neighbouring placement often collapses it instantly. And if two cells in the same row or box both show the identical pair — "2 or 8" and "2 or 8" — then 2 and 8 are spoken for, and you can strike both digits from every other cell in that unit. That is a naked pair, a genuine intermediate technique that shows up in the endgame of medium puzzles more often than you'd expect. The full technique guide lays the whole family out in order.

Writing notes is not lost time

"Pencilling everything in takes so long, I'd rather be solving." It feels that way, but the stopwatch disagrees. Without notes, medium grids force you to re-examine the same cells again and again, and the total time goes up, not down. Worse, holding candidates in your head guarantees a slip once you get tired. In practice, solvers who spend three or four minutes up front on notes finish faster and cleaner than those who dive in. Notes are simply the board made visible; more often than not, the moment you finish writing them you spot a cell that was already decided, which is a reward in itself.

A checklist for when you're stuck

When you grind to a halt, don't just stare harder. Work through this list in order.

• Did you erase everywhere? Check that the digit you just placed is gone from the notes in its row, its column and its box. This is by far the most common cause of a dead end.
• Is there a cell with a single candidate left? Sweep the notes from one corner to the other.
• Have you scanned all nine digits? "I never actually looked at the 7s" happens constantly.
• Is there a digit that fits only one cell in its box? Even a cell crowded with candidates is decided if it is the only home that digit has in the box.
• Have you walked away for five minutes? Make a cup of tea, come back, and the missing move is often embarrassingly obvious. Resetting a stuck viewpoint is a real effect, not a superstition.

If a full pass through that list changes nothing, the error is almost certainly somewhere in your notes rather than in the puzzle. Learning to suspect your own bookkeeping first is itself a step towards intermediate play.

When medium gets fast, move up

Four of the 20 daily puzzles sit at this level. As a rough marker: once you finish them reliably in ten to fifteen minutes using notes, your foundation is solid. When you want more resistance, hard sudoku introduces locked candidates and the rest of the intermediate toolkit. Every wall you climb hands you the next kind of fun.