I recommend the site recommended by Stephan Merz recently.
http://www.lwb.unibe.ch/
It is very good.
(I wanted to know if G ( x v y) <-> ( G x v G y ) )
And the anwer is...
... I let you look for it :)
Hint:
G ( x & y ) <-> ( G x & G y )
is true.
--
FL