While paper-surfing old books, I hit something that led me to the D’Ooge Key:
§368 page 159
1. a) Quis Caesarem iter facere iussit? – b) Quis imperavit ut Caesar iter faceret?
The translation task in the original book is:
”Who ordered Caesar to make the march? (Write this sentence both with impero and with iubeo.)”
In the main text of the chapter, an EXAMPLE of impero is given: ”The general ordered the soldiers ...