Question

What is the alignment of the sun moon and the earth that causes a lunar eclipse?

Answer 1

When everything (Sun, Moon, Earth.) are perfectly aligned, The sun being in front of the Earth, the Earth thus casting a shadow upon the moon hence causing the eclipse. 

Answer 2

When the Sun, Earth, and Moon are all in the same straight line,
you get a lunar eclipse.

Notice that the sun is ALWAYS "out in front" of the Earth, and the
Earth's shadow is always "behind" it. We get a lunar eclipse when
the Moon sails through the shadow.