What are the limitations in using a manual lens with a digital camera (nikon)?

I tried looking up online, but the response wasn't - shall i say, satisfactory. I'm using a Nikkor 50mm on my D5100 on M mode, my camera doesn't have an inbuilt AF motor, so I have to focus manually. That I have learnt, but the viewfinder zooms humungously and I have to step backwards to get the proper shot? Why does this happen? I think it's because of the sensor crop and all that, which happens when using lenses from a 35mm film camera to a DSLR?
Another issue i'm facing is when in Live View the view is just as it should be, perfectly lit and all, but when I hit the shutter, the image that comes is poorly lit and very dark? I then shoot at very low speeds, below 1/25 then obviously the shake screws it up. Can't seem to strike the right balance between aperture and shutter here.
So, basically, can anyone give me tips on how to get the best in this situation, if you've yourself been in it?
Also, the images are exactly they would be when shot with a film camera, which is a little less color and saturation but the sharpness is brilliant.

Added (1). I know 50mm is a prime lens, fhotoace, and I know it does not zoom at all and that is specifically why I mentioned the images are too in-the-face and look zoomed on their own as compared to putting the same lens on a film camera, it would have a wider coverage. That's what I meant. And yes, you sorted it out for me by saying it becomes a medium telephoto, that's what I wanted to know.

You really, really need to take at least one class in photography.

* 50 mm lenses are prime lenses so do NOT zoom at all. The reason you have to backup is because on your camera it is a medium telephoto lens meant for shooting portraits. Use your fine 18-55 mm lens for shooting landscapes, group shots and head to toe shots of people

* Live view is NOT meant to make your camera into an expensive P&S. It is used in the following ways: a) when attached to a telescope or microscope, shooting product shots in a studio with the camera attached to an AC adapter, when shooting shots of welding or other subjects that could damage your eyes if viewed directly through the viewfinder or when shooting video.

* Hand held shots should not be attempted at shutter speeds lower than 1/125th second until you have a lot of experience holding the camera super still during longer exposures For now use that ever aperture you need to get the correct exposure with your shutter speed set to 1/250th or faster for a while. If you can't open your lens aperture enough to get the proper exposure (using the light meter, increase the cameras ISO a couple of stops

List of compatible Nikkor lenses with various Nikon cameras

http://www.kenrockwell.com/...y-lens.htm

The limit is: YOU CAN'T AUTO FOCUS

sounds like you are too close when the image won't focus…

but, the camera/lens wouldn't make any noise if the lens is on manual, cos neither of them are doing any of the work.

because the lens is a 50mm prime lens, you would need to move back and forth to get the shot you want… But 1/25 is quite a fast shutter speed and you'd have to have a severe shake on to make the shot blurry…

Manual focus is very timely and not exactly easy. If you are shooting anything that is fast paced like kids you would miss a LOT of shots. If you are shooting fast paced you would be wise to use a high aperture to give your focus a large depth of field that will be in reasonably sharp focus. That is kind of defeating the purpose of a fast lens.
1.It isn't a true 50mm on a crop sensor camera. It's somewhere around a 75mm. Film is full frame. What you see in the viewfinder is what it really is now.

2. Live view the screen is brighter in a dark setting so that you can see. It is not an estimation of your exposure. Just as when you look through the viewfinder and see the scene it is not an estimation of the exposure. You have to create that exposure through your settings. If you are using one of the semi-auto modes your camera can't get enough light in and you need to change something: the lens to a faster lens (f/2.8 or more) or the ISO to a higher number. Set your shutter to the minimum you can use for whatever you are shooting. Don't touch it again. That is what you HAVE to have. Now if you aren't properly exposed you have to use one of the other elements. Change those instead. If you can't go to a wider aperture because your lens is maxed, then you have to change ISO. If you have maxed your ISO there's nothing else you can do but add light. With every lens and every camera at some point you have to add light.