Once you learn how to use them, through practice and experience, Aperture Priority and Shutter Priority Modes give you a much better chance of capturing that perfect photograph. But … they still depend on your camera’s ability to sense the right setting needed. Cameras have become very advanced, computerized devices that are quite good at what they do. However they are still a bit limited in some respects. They cannot sense nearly the range of color and light that the human eye can sense. So they have a way for you to override the camera’s choice. If you check your exposure as described above and it is not properly exposed, you can use the Exposure Compensation settings to tell the camera to allow more or less light to reach the sensor. Exposure Compensation allows you to adjust the light incrementally by fractions of an f-Stop to increase or decrease the amount of light that reaches the sensor.
If you are uncertain about what settings to use, exposure bracketing can be a good way to make sure you have a better chance of getting that great shot. It will also give you the possibility of composing an HDR image to expose both the extreme shadows and harshly lit spots in one image.