If you take on a shelter dog, the easiest way to train them is clicker training.....especially a mouthy adult dog that was either never trained or taken away from mum and littermates too early
Unsocialised dogs tend to react better to the clicker than just vocal commands, it still takes time and patience but the dog does curb any aggression or fear of humans much quicker.
Years ago when I worked alongside shelters and retrained their more aggressive dogs for rehoming, I came across a very old Alaskan Malamute.
She had been subject to a cruelty removal case, we estimated her age at around 12-14 years old. She was completely unsocialised, unhousetrained and covered in thick matts. She snarled and snapped at everything and everyone, which isn't natural behaviour for a Malamute. I sat in a corner of her kennel with her every day, all day for about 6 weeks, slowly building her trust. She was hard work added to that she was arthritic and almost blind. I could not bear to see this old lady all alone, so after we gained each other's trust I took her home, gave her very basic clicker training....mainly for her to let me know when she needed a pee or poo and to keep her calm around strangers. She stayed with me for a little over 2 years and passed away quietly in her sleep one night. She would never have lived that long had she been left in her old home and definitely not had she been left in the shelter kennel.
Some might say that a dog of her age and with her medical needs should have been euthanised, but I couldn't leave her to that fate, she deserved some love and affection in her twilight years, she deserved to know what it felt like to be loved. She would sit on my lap - which was interesting since Malamutes are far from lap dog size - rest her head on my shoulder and just quietly murmur to herself, she was a beautiful dog and she missed out on so much in her life, it was an honour and a pleasure to give her a good home for a few years, she was happy, she was comfortable and she was loved.
Visiting shelters is all too easy to go straight for the pups.....there are some awesome older dogs who deserve just as much love and cuddles as the pups but who get overlooked and ignored and to me that is so wrong cos that old mutt with its greying muzzle can give you as much unconditional love and affection as any tiny puppy can...sometimes more than a puppy.