Question: Is it possible for sun to come in between earth and moon ?

Like an eclipse when earth comes in between Sun and moon or when moon comes in between sun and earth .

What are conditions for Sun to come in between earth and moon ?

Answer: 
The diameter of the sun is 1,391,400 km and the moon is an average of 384,400 km away, so it doesn't fit in. 

You could ideally do it if the sun becomes a black hole: the sun has a [Schwarzschild radius](https://en.wikipedia.org/wiki/Schwarzschild_radius) of approximately 3.0 km, but gravitational forces make the system very unstable. 

It is more likely that the moon will go beyond the sun, but you need at least $1.7847\*10^{20}N$ to get the moon out of the orbit.
I do not think it is possible if not artificially.
As stated in the other answers, this is completely impossible for our earth-moon system. 

However, you could give your moon a very elliptic orbit and have it go behind the sun. This will cause several side effects (and it would probably make life on earth impossible):

* You would only see your "moon" for a few days every 10+ years (I
haven't done the math, feel free to correct me).
* Earth's orbit would be heavily affected. The mass of the moon is
non-negligible, so every passage would throw the Earth off his orbit.
* Tidal effects would probably wreck the surface of our planet.

And, even in those conditions, you would hardly have some spectacular effect when the sun "eclipses" your moon: just a minuscule dot, invisible to the naked eye, disappearing behind a gigantic object a hundred of millions times its volume.