How many times have you said these exact words? Better yet how many times have you said this to your dog, only to get a blank stare and the same old behavior you didn't want in the first place? Believe me, your dog heard you just fine, and so did everyone else around you, what your dog didn't do is UNDERSTAND you. Listening and comprehension are two very different things, and very frequently dog owners don't separate the two! Imagine if someone started a conversation with you at work and it went something like this.... "Hi, I need you to snorgle the badadonk tomato send email gargantuan to monkeybutt oh seventeen hours" After shaking your head in confusion, which of those words actually made any sense to you? "Send email", right? But it'll take you a moment to figure it out with all that excess stuff around those words, but you will eventually because those are words you hear regularly at work, so those are the ones that make sense. Now go back and remember what you said to you dog when you gave him a command, was it just one word, such as "Sit"? And did you do it in his "office" where the words are familiar or did you do it somewhere where he's never been like a park or someone else's home? This scenario happens every single day with dog owners and training, we often assume the dog knows what we want, when in reality all he hears in gibberish in an environment where he's never been, in a language he barely "speaks". That's what separates us from them in HOW we learn something, and assuming your dog learns in the same manner as you will only make you frustrated and your poor pup confused.
If they can't perform a task, ask yourself, "Is he listening or UNDERSTANDING?" Chances are it's the latter and almost never the former, so slow down, build a better foundation of skills and understanding and THEN the listening will happen :).