I can completely relate to this problem because I was also curious to play around with the camera settings when I first got hold of a professional camera my friend bought. The instance he showed me his camera, I got busy with various modes, controls and dials. My friend was pursuing a photography course from his school and I used to move around with him for all his assignments. As he used to shoot the pictures, he used to explain me the various settings. Sometimes he used to get specific assignments for just playing around with specific modes.
On one such assignments, he was shooting in the manual mode. And it was amazing that finally the camera was completely in our control. We tried out weird combos of shutter speed and aperture. Sometimes we used to get a complete dark picture and rarely could we get a picture with proper exposure. It was at that time that we realized that the manual mode was not our cup of tea.
My friend queried about the manual mode and its erratic behavior at his photography school. And this is what he explained me.
When the camera is in the auto modes, the camera meters the scene, reads the light and automatically sets the aperture, shutter speed and ISO. While working in semi-auto modes (aperture priority or shutter priority), even though we can control either aperture or shutter speed but the camera varies the other parameter, i.e if you are controlling the aperture, camera will set shutter speed itself and thus sets the exposure itself according to the meter reading. But when we are working in the manual mode, camera leaves everything on you, i.e, you have to set aperture, exposure, ISO all by yourself. You will have to meter the scene yourself, may be with the help of external light meter and set aperture, shutter speed and ISO accordingly to get the proper exposure. Any random values will not get you the proper exposure be it the widest aperture.