[jessy]

Hydrolysis products of sarin = (RS)-Propan-2-yl methylphosphonofluoridate are isopropyl methylphosphonate and methylphosphonate.
Which of them is the main product? I think that it is methylphosphonate, but I am not sure.

Can you help me please? 

[Arkcon]

I don't understand what you mean, "main product."  Sarin decomposes in the presence of water and alkali, (alkali meaning even short term exposure to an aluminum metal casing of a munitions shell,) into isopropyl methylphosphonic acid, which then breaks down into methylphosphonate.  So it all depends on when you're looking at it.