I'm reading Hunger Games. Katniss and Peeta are being prepared for the Arena by special instructors. The plant instructor says: Avoid berries unless you are 100 percent sure they aren't toxic.
How would you fill in the gaps?
Our plant instructor made a point of telling us to avoid berries unless you ____ 100 percent sure they _____ toxic.
This is a grammar point that most grammars don't cover. I don't know what grammar term is used for this grammar point and I don't know how the rule concerned is formulated.
In the novel I have the sentence in the proper form. But I would like to know how native speakers would fill in the gaps, and what term is used in English grammars.