@TianLian Really nice drawing! - I think the main things for me pulling the background forward hard edges and a tangent. The thinner tree in the center of the two main trees in the left foreground seems to want to belong to the background at the top and the foreground at the bottom - the tree could be curving back but I think it is helping to bring the rest of the trees on the right that are painted the same value and similar edge treatment forward - the bottom of that tree has harder edges and slightly thicker lines which brings it forward and as we near the top the lines fade a bit which help it go into the background - the bottom of that tree creates a tangent on the left too which is a way to bring an object forward that belongs in the background - there is nearly a tangent on its bottom right too. The hard edges on the outlines of the bushes has a flattening effect which also makes them seem to exist in the foreground - another way to bring the background forward is with saturation - your background colors are more saturated than the foreground colors which bring them foreword to our eye - making the foreground much darker will help bring it forward - this looks like it is not digital? could you make a few copies and mess around with value a bit on them? The tree in the foreground should be quite dark in places even if it were a white tree I'm thinking - anyways.. feel free to ignore - I really like your drawing!