Hannah, based on your question and your statements you seem to not have an understanding of the mechanics of a camera and how to control them. I am not judging, No one is born knowing this, we all have to learn it.
The brightness of the image you see in the view finder is not the brightness or exposure that the camera is going to record. It shows you a bright image so you can focus and compose your shot. Some cameras have live view on the LCD screen, this lets you see the exposure as it would be with the setting that have been chosen on your camera. This may be why you were getting the expected exposure while using the LCD and why you may have wrongly expected the same to be true of the viewfinder.
I did a you tube search for - Beginners guide to DSLR cameras - and - the mechanics of aperture and shutter - and got many good tutorials. I would recommend you spend some time watching as many as you can. If you are interested in photography as more than just a hobby i would advise you to volunteer to intern with a working pro photographer and take some classes.
Photography is all about light. There are TWO way you can control the amount of light that enters your camera.
Aperture, the size of the opening in your lens.
Shutter speed, the amount of time that the cameras shutter is open.
ISO also affects exposure but it dose NOT change the amount of light entering the camera it only changes how sensitive the sensor is to the amount of light your are letting in and how it records it.
Your job as a photographer is to understand light, how your camera sees and records light, how to change the settings on your camera to record the light in a scene and how to shape and create light in order to express your artistic vision of what you are trying to say with your photography.
You can not drive a car until some shows you what all the pedals, buttons, wheels are for and how to use them. Your camera is a complicated tool and it has some intelligence programed into it but your brain is far more complex and you will need to override your cameras brain in many situations so feed your brain.
Here are a few links to videos i found.
this one has an interesting graphic that is informative.