- Disambiguation: For imperative in the sense of a countable noun, i.e. "an imperative" see command.
- "Stand up!"
There is however an implied subject - that is the second person, e.g. "You stand up!" - but this is usually omitted.
The imperative is used for giving instructions, including in the classroom. For example, "Open your books! Turn to page 69! Read the text!" etc. In real life it's also used for other instructions, e.g. instruction manuals, recipes, etc.