Why dont eclipses happen twice a month?

Eclipses would happen twice a month if the moon's orbit were in the same plane as the Earth's orbit around the sun. The moon's orbit is slightly tilted, so eclipses are less frequent.

The plane of the moon´s orbit is inclined 5 degrees to the plane of the earth´s orbit around the sun. this being the case the moon´s orbit interesects the plane of the ecliptic at two different points (nodes). when one of these nodes is lined up with the sun and the moon passes through one of these nodes there is a solar eclipse. if the moon is at it´s apogee when this happens we have what´s called an annular eclipse because the moon´s disc does not completely block out the disc of the sun. when the moon is at there is a total eclipse.