To see Mercury with your naked eye is only possible when it is at its greatest "elongation" or basically when the angle of separation between the Sun or Mercury is at it's greatest value, however it is not always a promised success of viewing Mercury. It also depends on the phase of the plant to determine how bright it is!
Mercury is hard to see from Earth because it's so close to the bright sun. It is the closest planet to the sun and the smallest of all the 8 big planets. Mercury is only a little bigger than Earth's moon.
