A dog with a soft mouth is a very good thing. It means that the dog retrieves birds without biting down on them so they are in good condition for the table. A retriever should bring birds to hand without biting, and it should bring crippled birds back alive. The opposite of a soft mouth is a hard mouth. Hard-mouthed dogs bite down on birds, sometimes only leaving a tooth mark or two, sometimes crushing or mangling the bird so it’s practically inedible. A hard mouth can be an inherited trait in some dogs, or a puppy can become hard mouthed if it has a bad early experience like getting thrashed or spurred by an injured bird. The best way to ensure your dog has a hard mouth is to do some sort of trained or forced retrieve training even if the dog has a strong natural retrieving instinct.