actually, there is a dedicated imperative mode in German, which would be schreib(e)
This is just a statement.
It could be used as an order, like making an order by making a statement. (I think it is similar in English?)
'Du schreibst jetzt den Brief!' 'Du schreibst den Brief jetzt!' 'You write the letter now!'
'Du schreibst gerade(/jetzt) den Brief.' 'You are writing the letter.'