Minesweeper patterns and strategy
Four situations that keep coming back. In each figure, the flag is a mine you have already decided, and the empty light square is safe.
1-2-1
Three numbers, 1 then 2 then 1, against a row of three closed squares. The 2 needs two mines among those three. Each 1 needs one, and the two 1s share the middle square with the 2.
The only count that works is mine, safe, mine. The mines sit over the 1s, and the square over the 2 is safe. On medium this shape shows up all the time, on the edge of a clearing.
1-2-2-1
Four closed squares over the numbers 1, 2, 2, 1. The first 2 sees the first three and needs two mines. The 1 on the left sees only the first two and needs one. The difference is the third square: it is a mine. The same reasoning, from the right, marks the second square as a mine.
The two ends, over each 1, are safe. The two mines sit over the two 2s.
The corner
A square in the corner of the board sees only three neighbours. If its number is 3, all three are mines. If it is 2, you still have to decide which of the three is safe: do not flag all three.
The same idea applies to a square on the edge, which sees five neighbours. A 5 on the edge closes all of them. A 4 still leaves one safe.
Counting the mines left
The number at the top shows how many mines are left. It starts at the total and drops with each flag. When a closed-off corner has exactly that many squares, all of them are mines. If the number reaches zero, flag nothing else: what remains is safe.
That settles the end of a game in one step, especially on hard and on the large boards, where a corner is left far from the region you were working in.
When you are stuck
Do not guess. Walk the numbers you have already opened, starting with the ones you opened last and the ones that sit side by side. The certain move is on one of them.
To review the click, the flag and the long-press, the rules page has the computer and phone controls.