When I take photos on my Nikon D3300, unless it's in auto mode, the photos are always incredibly bad quality and too dark, even though the ISO is on the highest. What could be the cause of this and how can I fix it? This only happens when I take photos, videos are perfectly fine
"Incredibly bad" might be because you are using too high ISO. Use Manual Mode, open up the aperture to the widest, set the shutter speed to 1/100 and then adjust the ISO as necessary. And use the viewfinder while taking stills. Quality of the photos taken in LV is not WYSIWG (not at least with D5100, which I use).
The more the higher ISO, more will be the noise in image. Always, try keeping the ISO at minimum level as possible. Use wide aperture and shutter speed of suitable speed to capture photos. If you are new to photography, don't directly jump into manual mode. Try aperture, shutter priority mode first. As a first, learn the triangle of photography: aperture, shutter and ISO and keep experimenting with them. Good Luck!