The most useful key in Lightroom is the ALT key (Option Key for Mac users). In the exposure settings, holding the ALT key while moving the sliders will show you when you over or under expose some areas of the photo. But it is especially useful when you sharpen the image. If you hold the ALT key while moving the Masking Slider, you will see your mask on screen. The sharpening settings will be applied to the areas in white while the black area is not affected.