New to Sudoku? The rules, your first number, and what the red means.
Your first solving technique: find the cell where only one number can fit.
A number with only one place left to go — scan the row and it reveals itself.
A number trapped on one line of a box can't appear anywhere else on that line.
When a row traps a number inside one box, the box gives up its other options.
Two cells sharing the same two numbers lock each other in — and free the rest of the row.
Two numbers with nowhere else to go claim two cells — even while those cells still look open to others.
Three cells sharing three numbers between them — even though not one of them shows all three.
The mirror of the last one: three numbers that have run out of room, sharing three cells between them.
The first pattern that spans the whole board — one number, two lines, four corners.
The X-Wing's cousin: two lines of one number sharing a single crossing line, their offset tops ganging up on a fourth square.
Three two-candidate squares chained across the board — a pivot and two pincers gang up on a fourth.