How to play Minesweeper
The board starts covered, with mines hidden somewhere under it. Uncover a square and one of two things happens: it holds a mine, or it shows a number saying how many of the eight squares touching it are mines. Your job is to work out where the mines are by counting, and clear everything else.
A square with no mines around it uncovers its neighbours automatically, and any of those with nothing around them do the same in turn. That is why one click can open half a board at a stroke.
You win by uncovering every square that is not a mine. You never have to uncover the mines themselves, and you never have to mark them — marking is a note to yourself, not a move the game requires.
The two deductions that do everything
That is the first deduction: a number whose covered neighbours have come down to the number itself. Every one of them is a mine.
The second is its mirror: a number whose mines are all already accounted for. If a 1 is touching a square you have proved is a mine, then every other square touching that 1 is safe, and you can uncover all of them without thinking about them again.
Nearly every board is these two moves, alternating. When neither one works where you are looking, the answer is usually two squares away — a number on the far side of a wall you have not read yet.
Why you are never asked to guess
The classic game has a flaw, and it always shows up at the end. Two covered squares, one mine between them, and nothing anywhere on the board that tells you which is which. Players call it a fifty-fifty. Forty careful moves, and then a coin toss.
Every board here is checked before it reaches you. A solver plays it through using only what you could know at that point in the game. If it reaches a position where no square can be deduced, the board is not handed over: a mine beside the sticking point is moved, the check runs again, and a board that keeps failing is thrown away for a new one.
So when you are stuck, what you need is already on the screen. More on how the no-guess check works, and what a fifty-fifty actually looks like.
Playing here
You never start from nothing: the board arrives with a safe square already uncovered and the automatic clearing already run out around it. Dig with a left click, mark with a right click, or switch between the two with the Dig and Flag tabs if you are on a phone.
Hitting a mine stops the board rather than ending the game. Press Undo and you are back where you were, one move earlier. A board you have been reading for ten minutes should not be lost to one slip of the hand.