If the hardwood is in good shape and you don't want to have to sand and refinish, I would recommend covering with the floor with red rosin paper (tape the seams to keep it in place), then a layer of carpet padding and finally a layer of 1/2 plywood or OSB. This will protect the floor from scratches and dents should any larger chunks of wood, plaster or drywall (or tools) fall on the floor.
They make a material you can paint on and when it cures it forms a rubberized barrier that goes a long way toward protecting floors. The above approach works as well.
And here is one some sub-contractors fight me on tooth and nail at times. I have a keep clean clause or two embedded in all my contracts. I expect contractors to bid the time it takes to sweep up after themselves (not just plunking stuff down floor vents) into their work for me and to perform such tasks so the jobsite stays relatively clean and safer. You'd think their mothers were asking them to straighten up their rooms or something. But if the nails are swept from the floor they are not going to get ground into it as people walk all over things.
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