"It will be necessary that the door has been opened before."
Translation:Il faudra qu'on ait ouvert la porte avant.
why does the 'answer' use completely different words than are offered in the exercise? How is one to learn from this?
You have already learned that "il faut que" means "it is necessary to/that" in earlier lessons.
This is only the same case but in the future tense: "it will be necessary that/to" = "il faudra que" is part of the best translation.
And all of the usual variants are valid:
- il sera nécessaire qu'on ait/que nous ayons ouvert la porte avant
- il sera nécessaire que la porte ait été ouverte avant (passive)
- la porte devra avoir été ouverte avant (passive)
- nous devrons / on devra avoir ouvert la porte avant
I understand that DL is testing verbs, but this sentence in English is both unnatural and meaningless. Better choice: "It will be necessary that the door must be opened before the concert begins."
Alternative sentence which actually means something in English: 'It will be necessary that the meal has been finished before leaving'. Even then, I wouldn't say it but it targets the necessary points of grammar.