A unit in a forest counts as being in soft cover provided that the majority of its models are within the forest. Similarly, if a model in a shooting unit has to trace its line of fire through a forest (because it's shooting at a target beyond the forest), then the target benefits from soft cover. This applies even if during the game you rearranged a forest's trees around a unit, or even removed them – just imagine the trees are still in their original position. This penalty does not apply if the shooting model is drawing a line of sight out of a forest it is in.