Intermediate moves to a 16×16 board with 40 mines — big enough that guessing by feel stops working and real flag discipline starts to matter. You'll see the 1-2 pattern often: a 1 and a 2 sharing two hidden neighbors, which pins down a mine the moment you compare them. Keeping the mine counter in view as you flag helps you catch a wrong flag before it costs you the board, since a count that's already gone negative means one of your flags is wrong somewhere on the board. New to the rules? See how minesweeper works.
Too much? Beginner is a step down. Comfortable here? Expert is a step up.