Novice! My SLR is Nikon D5 100, and the photos taken are either too dark or too bright! !

Two suggestions allow you to control the exposure:

1. The priority of using aperture is to set the dial to "A". If the subject is bright, such as daytime and outdoor, the aperture value can be lowered by another small gear dial (such as F8, the larger the number, the smaller the aperture), and it can be raised indoors or in a dark place (such as F3.5). In this way, the camera will automatically adjust the exposure time according to the measured value.

2. Make good use of measurement mode. General point metering, central button metering, or surface metering. If the contrast of the scenery you shoot is too big, such as shooting people against the light, if you point the light at people, you will shoot clearly, but the scenery behind you is overexposed. If you don't want to overexpose, you need to use surface photometry at this time. You will feel these skills when you practice. Just shoot more and think more.

About pure theoretical knowledge, you can search for it yourself, there are many. Personally, I still need to practice more.