According to Vet Info, a dog's memory is short, lasting only around 20 seconds. To remember things, dogs rely on associative memory more often than real memory.
For example, if a leash is used to walk a dog every time the dog is walked, it eventually associates a leash with going on a walk. Associative memory can also have a negative impact on a dog. One example of this is if a dog gets hurt during a routine nail clipping and then associates nail clipping with pain. Both positive and negative associative memories can last for many years in dogs.