why we don't have solar eclipse every month?
The moon's orbit is at a slight angle from the plane of earth's orbit around the sun, so it doesn't always hit the right spot, as described on NASA's Total Solar Eclipse 2017 site:


  Eclipses only occur if the Moon is located within 0.5 degrees of the
  plane of the ecliptic, on a line that passes through the center of the
  Sun and the Earth. The Moon travels along an orbit that is inclined by
  5 degrees to the ecliptic plane, so there are only two opportunities
  each month when it passes through the plane of the ecliptic. These
  points are called the ascending and descending nodes. Eclipses of the
  Sun only occur if new moon occurs when the Moon is near of one of
  these nodes. A similar argument explains why lunar eclipses do not
  occur every full moon at the node opposite the Sun from the Earth.