It is small wonder that dogs are often confused when given commands by their owners. How can a canine mind work out that "Sit!" means the same as "Will you sit down for heaven's sake?!" or "Come when I call!" means the same as "Here!"? When in training and indeed afterwards it is very important to use exactly the same words, preferably of single syllable and sharp in tone, prefaced with the dog's name: "Blazer here", not "Come when I call you".
The vocabulary should consist of eight, maximum ten commands and never vary in content. Then the dog knows where he or she stands. Regrettably children do not follow this rule and the dog gets very confused.
A good idea is to give commands in another language than your own, one which is totally alien to you. Anyone can learn ten words in a language within half an hour but you do not know the variant.
When in Egypt I inherited a German Shepherd that had been trained by the local police but had failed her final test so was in need of a new home. I needed a guard dog so I learned ten words in Arabic. It worked a treat as I knew only those words and did not know their variants.
What is the most important command a dog should learn? In my opinion it is "Here" or "Come", one or the other and never vary. All kinds of situations occur and your dog may want to investigate or play. Often this is to his or her peril. You need to keep the dog close to deal with the situation. The dog is not in his or her natural world but a human world with human perils for dog and people. The owner is equipped to deal with the situation.