Suppose the moon were closer to Earth. How would the force between Earth and the moon be different?
For more details, look up "Newton's law of universal gravitation" on Wikipedia, but the gist is that the force between the Earth and the Moon depends on the product of the masses divided by the square of the distance, and then thetre's a factor g that is constant.
Taking a simple case, suppose you have two masses weighing 100 and 30, separated by a distance 8. The force between them will be proportional to
(100 * 30) / ( 8 * 8)
or 3000/64, about 47
Now suppose we move them closer, to a distance of 6. Now the force is proportional to
(100 * 30) / ( 6 * 6)
or 3000/36, about 83.
At a distance of 4, that works out as 3000 / 16, or 187.
Halving the distance quadruples the force! This is because the force depends on the _square_ of the distance.
What effects does the force of gravity between the moon and the earth have on the earth? (Starting Hint: think water.)
Now you should be ready to tackle the question!