I have a Nikon N75 SLR film camera whick I will keep.Can I buy a Nikon D40 SLR digital camera and use the lenses with my N75 and the D40? Yes they are Nikon and Nikkor AF lenses. In case you ask the N75 is an F body that accepts G or D type AF lenses. As Ben pointed out, the Nikon D40 is an unique animal in Nikon's D-SLR catalogue in that it can only autofocus with the newer "AF-S" lenses.
You will be able to manually focus your Nikkors on a D40.
You would be wiser to look at the Nikon D80 or D200 cameras which can autofocus with AF lenses. You might also find older D50 or D70 cameras available - and they can also autofocus with AF lenses.
It sounds like you may have some manual focus lenses as well. Backward compatibility of the Nikon "F" mount is general. For manual focus lenses, research them specifically for compatibility - some may not light-meter correctly with the newest Nikon cameras.
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Disclosure: I am the owner of www.lenslenders.com in Canada. The D40 will only autofocus with AF-S and AF-I lenses. These are the one which have an integrated focusing motor rather than being driven by a screwdriver in the lens mount.
Older AF lenses will still mount and work fine, however they will be manual focus only. I agree with lenslenders. Since you already have a few lenses, you might not need to buy any now, so just getting a D80 body would not be cost prohibitive and the rewards would be well worth it.
If you do get a D40, you'll probably be stuck buying it as a kit with the 18-55 lens anyhow. You can't auto-focus most (or all) of your lenses, but you CAN get a focus confirmation with most of them. From the D40 manual: "If the lens has a maximum aperture of f/5.6 of faster, the viewfinder focus indicator can be used to confirm whether the portion of the subject in the selected focus area is in focus. After positioning the subject in the active focus area, press the shutter release button halfway and rotate the lens focusing ring until the in-focus indicator is displayed." | 