I recently bought a Nikon FG, and when i look through the viewfinder all i see is black. I made sure the lens cap it's on, and its still black! I need to replace the batteries, but will that fix the blackness?

I am assuming it is a TTL (Through The Lens) Viewfinder meaning a mirror, Prism and screen gives you the image. This is based on reality and not any settings. This question confuses me. If something gets in the way and the viewfinder darkens AND the image you take is dark this means there is something very wrong with the lens and possibly with the camera. The Viewfinder image is what bounces off a mirror and when you take the photo the mirror flips out of the way. What is left is the lens, some air space and the sensor. It may be that one of the aperture blades may be moving when it should not. I am close to saying send the lens in and ask for a quote..

No, you shouldn't. The D70 works as SLRs have worked for half a century, allowing you to compose your photos through the viewfinder. The ability to use a dSLR as a point-n-shoot camera, wobbling at the end of your arms, is a relatively recent development. Nikon introduced it into their lineup with the D3 and D300, released years after the D70.

Your mirror is probably stuck in the upward position. If so, your camera will need servicing.

Have you removed your lens cap? Have you turned it off? Does the small viewfinder work? if so go to the menu,,, not sure on your model if it has a separate button to turn it on and off, look in your manual for EVF

The D40 works the same way SLRs have worked for the past half century. Only recently have dSLRs acquired a "Live-View" capability, where you see the image on the LCD monitor instead of through the viewfinder.

I realize this probably wasn't the answer you wanted to see, but that's the way it is.

Remove the lens and look inside it. Do you see the mirror? If not, your mirror is stuck in the up position. Its flat up against the focusing screen. If the mirror is down, the problem is in the lens, or you still have the lens cap in place.
Check your manual under troubleshooting.
Try setting the shutter speed dial to the M90 position.
Also be sure the batteries are inserted correctly!!!!

If no change yet, you can try to put the mirror back down yourself. With the lens removed, set the camera on its back. You can use your fingers, but I suggest a toothpick cuz they won't leave fingerprints on the mirror and the toothpick should break before you exert enough pressure to really break something. So just try to return the mirror to the 45 degree angle within the mirror box. Be careful not to damage anything in there, like the shutter or the arms that raise and lower the mirror. Depending on where you live, there may be some corrosion or rust on the mirror linkage. It should move easily. If you feel any roughness when you move the miror, that may be the reason it did not return on its own. Then turn it on, off again. At some point, you may want to have this camera cleaned, lubricated and Adjusted(CLA) - after all, its over 25 yrs old.
If all that did not work, you'll need to send it in. Here are some good choices for repair places...
Garry Airapetov in Niles, ILL
Camera Repairs in Avon, IN.
BTW here's a website with several pages about FG...http://www.mir.com.my/rb/photography/hardwares/classics/emfgfg20/fg/index.htm
copy and paste it into your browser.

looks like a viewfinder has an issue. If you have recently bought the thing just return it and get a new one. If not possible to replace , i recommend visiting Nikon authorised service center.
Could be something in the path of pentaprism.

If its black when you look thru the viewfinder its likely because the mirror is stuck in the up position. It will often do that if the batteries are exhausted. Try replacing the batteries.

The mirror might release back down if you turn the shutter speed dial to M90. This is a manual release of 1/90 second for when you need the shot but don't have any power.

When you look in the lens opening and adjust the aperture dial, you are seeing the aperture open and close. Not the shutter. You need to open the back of the camera (without film) and take a photo to see the shutter curtains open and close.

You should have a read thru the manual to become familiar with the camera and its operation:

http://www.cameramanuals.org/nikon_pdf/nikon_fg.pdf

By "stayed up", do you mean that the mirror remained in the up position so that you could not see anything in the viewfinder or do you mean that the shutter actually remained open? I do not remember the exact features on your camera, but in general, this is what I would do.

1) Remove the film from the camera and take a picture. Note if you see anything in the viewfinder. If you see nothing, the mirror is in the up position. Verify this by removing the lens and opening the door on the camera back. Look through the lens hole. If you can see through the rectangular hole at the film plane, the shutter is open. If instead you something is obstructing the hole, the shutter is closed. In any case DO NOT TOUCH THE SHUTTER! The shutter is what is covering the hole.

2) If the mirror is in the up position, check to see if your camera has a feature that allows locking the mirror in the up position. Some cameras have this feature to allow use of lenses with very short focal lengths. If this is the case, simply unlock the mirror.

3) If the shutter is the problem, your shutter speed may be set to "T", which stands for time. In the time exposure mode, you press the shutter release once to open the shutter and a second time to close the shutter. Cameras with a "T" setting also have a "B" setting, which stands for bulb. This is a throwback to the old days when it was common to use air-powered shutter releases rather then cable releases. The bulb was a rubber bulb that you squeezed to force air through a tube and push a pin to activate the shutter. The "B" setting keeps the shutter open as long as you hold the shutter release in, but as soon as you take your finger off the shutter release, the shutter closes. Both of these settings are used to make timed exposures. If you find that the problem was that the shutter was set to "T", set the shutter speed to 1/25th second or so and try again. The following sequence should occur; the mirror will flip up and the viewfinder image will disappear, the shutter will open for the prescribed time and close, the mirror will return to the down position and you will again see through the viewfinder.

If this does not work, you may need to take it in for repair.

The mirror motor is probably the problem, you'll usually hear a whirring noise like a motor spinning. It's about a $125 repair, requiring the mirror box to be removed and the mirror driving motor replaced. Pentax originally put a plastic gear on the motor shaft - the replacements have a brass gear.