What would happen if the distance between the earth and the moon decreased
The gravitational force between earth and moon will increase. by F = GMm/r^2 F is inversely proportional to r^2 when r decrease, F will increase.
The moon would appear much larger in the sky. the moon directly affects the ocean tides. If it were much closer, the ocean would rise and flood coastal areas. If the moon were much farther away from Earth, then the ocean tides would quit and the oceans would stagnate, killing all plant and marine life.