I need help with the lighting.. I am trying to make a semi-dark map and when i used light_environment, i'm not sure what to do. What does diffuse bightness and all that mean?? Please help. I also need help with the attributes on the light entity.

remember that the light env is NOT a "sun" or a "bulb", it is only an info file on how brushes with SKY on ALL sides will give off light!

diffuse = color of shade. ever notice that shade on snow looks kinda another color, like blue?

pitch is up or down angle. 90 = straight up from ground, -90 straight down from sky (north africa or equator at noon). for europe a setting of -30(morning/afternoon) to -60(noonish) is good. also good settings for the moon. (if you want to play with an annalemma, time & date you can get the exact sun pitch for a historical battle.....)

angle - which side of the map the light (sun & moon) is coming from.

ok, you want it darkish. night? make light grey/blue for moon light, and brightness of 15 to 30ish. dark. then set
HLRAD -extra -bounce 1 -smooth 80 -coring 0.2 yourmapname.map
bounce 1 or 0 makes shadows darker for inky night shadows. coring helps night lights have less "bullseyes".

overcast day? grey agian, brightness 50-80ish. set
HLRAD -extra -bounce 9 -smooth 80 yourmapname.map
more bounce = fainter shadows, grey days have weak shadows.

What about diffuse brightness? Leave it as is? and How do I get hlrad to work in hammer 3.5?

dont use hammer to compile use a batch compiler, It will give you all the info you need on it site, get the one in the mapping files sticky post.

Diffuse brightness is like an environment light that is not affected by world brushes, it basically evenly disributes light on the level, effectively creating your "shadow" color. Play around with it the get different effects like adding color to shadows, but generally you can just leave it as is.