Wolfshadw :
99% sure you won't be able to use it forever and 100% sure it won't automatically revert back to your old OS. Shortly after Windows 10 is released, you will need to either purchase a license for it or re-install your older OS and programs.
-Wolf sends
i wouldn't assume that you will have to pay for it. Free for the first year sounds exactly like what it says. Unless they curveball ppl and turn it into a subscription service after the 1st year.
Just found this on PC Gamer http://www.pcgamer.com/microsoft-windows-10-will-not-be-sold-as-a-subscription/
and this from MS http://blogs.windows.com/bloggingwindows/2015/01/21/the-next-generation-of-windows-windows-10/
It will be free for the first year, no strings attached.