Translation:It is necessary to wait until he has finished sleeping.
"finir de" has a specific meaning to stop doing something when paired with the infinitive. So in English we naturally assume someone wakes up when they stop sleeping. Nothing wrong with this but technically someone may not be sleeping but is also not in a state of waking up - the two aren't necessarily interchangeable.
I agree that the translation in English is more flexible, but if we are being "literal" about the translation from french, the meaning with finir de is to finish sleeping in this case. Perfectly logical.
I'm confused, is this present tense or past? I know it's a past tense conjugation (passe composes) but duolingo is taking both present and past meanings as correct: it is necessary to wait until he has finished sleeping, and it is necessary to wait until he had finished sleeping.
This one is vexing. It'd be helpful if someone can explain why 'until' isn't translated in this construction. My understanding of 'attendre' = to wait for. So does 'attendre que' = to wait until. In an exercise for subj. doesn't ''qu'il " form part of the subject? To Duolingo.
I did scan the comments though I didn't find an explanation.
In my mind (as a native English speaker trying to perfect my French), it would be more logical to translate this sentence as, "Il faut attendre jusqu'à ce qu'il ait fini de dormir." Is this also a possible translation? Does it sound better/worse, more natural/less natural than the given sentence?
DL gives as a correct answer "We have to wait till he was done sleeping." English speakers, please clarify why "was"? I had written "till he's done sleeping" and that was considered an error. "Il faut attendre" is present tense and the waking up is going to happen some time in the future rather than in the past — to me this makes no sense.
The English translation I'd use is:
"We must wait until he has finished sleeping"
The French requires the subjunctive - "ait" form of Avoir (see other comments).
Re the English, your contraction ("he's") in full form is "he has", so "done" isn't correct here and is an adverb, whereas "[has] finished" is a verb. They aren't directly interchangeable imo.
Why the "has finished" construction? Although he started sleeping in the past, the fact that he may awake at any moment brings this action into the present. It's not happening in the past or future so using present perfect tense shows its relevance to the present.
For the past subjunctive: When do you choose between
present subjunctive + participle + participle (eg. il ait ete prepare)
present subjunctive + participle + de + infinitive (eg. il ait fini de dormir)
vs (if it is even possible)
present subjunctive + participle + infinitive