Children are asked in the SPaG to get the right example of a 'command'. The only one allowed is the one that uses the 'imperative' form of a verb - like 'go' as in 'go now', or 'get out' as in 'get out' (!). Easy to practice? Yes. Easy to learn? Fairly.
But is it true? Is it useful? Does it explain language-in-use?
Because of real language in use.
We can 'command' in various ways in English:
We can say, 'you must' do something now, or 'you have to'. Or even 'I want you to...' do something, if said with sufficient force.
We are also surrounded with signs like 'No smoking' and 'Quiet please' which are also 'commands'. But not SPaG 'commands'.
Even more odd are 'imperatives' which are not commands!
My favourite is the word 'STOP' on the stop button on a bus. It is not telling you to stop. You read it. But you can't 'stop'. It means, 'press this button and a message will go the driver who will, if he or she sees it, stop the bus.' Or it means 'This is the stop button'. Either way it doesn't 'mean' that you who are reading this must stop.
UPDATE: someone has just given me another example taken straight from teachers' and children's daily experience, daily real use of language: when teachers say, 'Sitting quietly' or 'Looking this way'. Yes! It's just the 'present participle' of the verb doing the work, the '-ing word'.
So teachers are using this as a command but it's not a 'command' command. It's only a command.