Scratches and dents -- what actually happens
The polyurethane coating on a finished timber floor is about 0.1mm thick. Most of the time what people call "scratches from the dog" are actually indentations in the wood itself, not damage to the coating. The timber species controls how well a floor handles this. Hardwood species vary a lot in density and resistance to impact, but generally hardwoods cop it better than softwoods, which are softer and mark easily.
No finish system can make timber harder or prevent physical damage from furniture, dropped items, or dog claws. Bona finishes will bend and follow the indentation though. If you look closely you can often feel the dent with a fingernail but see that there is no break in the finish -- the wood is still protected against spillage, wear and scuffing.
A dog walked regularly on pavements with regular claw trimming generally leaves few marks. But a heavy dog with sharp claws can have a point loading of many kg per mm, which can affect the wood. With all dogs, try to cut down on indoor play. Throwing a ball down the hallway or letting the dog chase around the sofa is fun, but it increases the chances of damage to the floor.
Spillage, urine, and flea sprays
Food and water bowls are the obvious one. Ideally they are not sitting on your timber floor, but if they have to be then get a decent rubber mat that catches the splashes. Be careful though -- some rubber and plastics can bleed plasticisers that mark the floor. Ask when you buy whether that is a known issue with the particular mat.
Puppies create problems with toilet training, and persistent urine in the same spot is a real risk. If you clean it up reasonably quickly, use an enzyme spray rather than bleach or vinegar, and stop the dog going back to the same place, you will generally be fine. Left for a long time, or with repeated hits on the same spot, dog urine can pass through all types of floor finishes into the wood below. This causes dark patches where chemicals in the urine react with the timber. Where dogs go on a mat, the mat holds the urine against the floor and can cause a more serious issue, often with dye from the mat releasing into the flooring as well.
Flea sprays are another thing to watch. These products usually contain a high proportion of aromatic solvents -- the idea being that the solvents help the active ingredients penetrate through the dog's skin. If you get overspray, or the dog shakes hard after treatment, the spray droplets land on the coated floor. Occasionally these drops chemically attack the polyurethane, leaving small white marks or blisters. Other chemical sprays with volatile solvent carriers -- insect spray, fly spray, leather polish, window cleaners, air fresheners -- can do the same thing.
Dirt, hair and drool
All three are part of having a dog. Your dog will bring dirt and water into the house, especially if it treats the garden like a racetrack. Good mats inside and outside at external doors are a must, particularly if you have a doggy door. If you can catch them and wipe paws with a towel when it is wet outside, so much the better.
Some dogs shed a lot of hair, but even breeds that lose very little will always leave some on the floor. Your dog probably has a favourite spot where it lies directly on the timber. Dog hair can be quite abrasive and in some cases can dull the surface of the floor. Keep it under control to prevent damage. That dull patch in front of the window might be down to your dog constantly rolling and turning in the sun.
If you have a bulldog or mastiff, nobody needs to tell you about the drool. Wipe it up and there will be no issues. But if there is a favourite drooling spot, over time you may see some staining of the surface.
Living with it and planning ahead
With some understanding of what dogs do to timber floors, you can take steps to protect your floor against some of it and reduce the effects of the rest. The floor will show some wear over time -- that is unavoidable. But if you take some basic steps to reduce the impacts, the floor will perform well.
Regular maintenance and planning for an overcoat of the floor every few years will keep things in good condition. Budget for recoating every 5-7 years with dogs, rather than the usual 7-10. Water-based poly in matt or extra matt sheen hides micro-scratches and paw marks better than satin or gloss. Oil finishes show marks more easily but can be spot-repaired, which poly cannot.