I answered, "Ellos firman el libro" and it was marked as incorrect. The correct answer was "Ellos signan el libro." What's up with that?
Apparently they have changed their mind.
"Ellos firman el libro" is now the primary answer.
Especially becaus they JUST told us in this same lesson to use firme
I used firma, that too was wrong.
Why do you need firman instead of firme?
yo firmo tú firmas él firma nosotros firmamos vosotros firmáis ellos firman