Assuming the proper size adapter, can you use a telephoto lens from a 35 mm film camera on a digital camera?

Yes, an old regular telephoto lens can be used with for example Canon EOS digital Rebel camera without a problem if it fits properly. An adapter is not necessary if they fit together.
The quality is good.
You should take your lens to your nearby camera store and try it before buying expensive camera.
Good luck.

they make digital cameras that work that way, yes. They are not cheap.

Yes. My brother did that. Yo ucan use any standard 35 mm lens fro ma regular camera onto a 35 mm digital camera.

if the digital camera is an SLR, if it's just a point and shoot, there is no way to do it. I have a SLR film camera and digital, the lenses are interchangeable!

In additon to a an adapter, you make also need a stepping ring to match the lens

Although more information is needed to give you a more informative answer. You can find a detachable lens for a digital camera, if you are looking for more zoom for the money. a good lens isn't cheap. You know most digital camera's already have a zoom option, right? Well, if you don't want to throw money away , hand over fist. You may want to consider purchasing a new camera. with more options available. I did a lot of research before purchasing cameras.
All the information rides on , what you want to achieve with the art of taking pictures.
There are some Camera stores that will let you trade up. So you have another option. Hope this helps.

No problem. Of course, you will not get autofocus, but with the right ring adapter you can use any lens form film camera in a digital SLR camera.

If they're the same mount, yes. And autofocus will work.