In Understanding Clauses and Complex Sentences, we learned that clauses always have a 'subject' and a 'verb group'. But here we are saying that commands only consist of a verb group. What has happened to the 'subject'?
The 'subject' is usually the person or thing that does the action. In a command like "Pay attention!", who is paying attention (or is at least meant to be)?
The 'subject' is whoever is being spoken to (in other words, "you"). So it's usually just implied:
"(You) Pay attention!"
But it can be spelled out:
"Everyone, pay attention!"
"Pay attention, Ginnie."
(It's worth noting that these are arguably not 'subjects' in the strict grammatical sense, which is why they get separated out by commas. But functionally, they specify the intended 'actor', which is what a subject would usually do.)