Mahjong tiles have to be clear on the left or right side; not touching a tile on one of their sides to be able to be selected. If you can select, the matching tiles, then they are free. It is occasionally sticky about allowing you to select tiles. They can be surrounded on 3 sides as long as a left or right side is free. The stars are there life system; if you quit in a level or get the no moves screen, you lose a life or a heart. Hearts regenerate with time.