Duoling rejects "He must know how to swim," even though the Conjectural Future Tense makes more sense here.
It also rejects "will be able to swim", while this phrase is even offered as a translation hint for "sapra" (with the accent). Can someone either clarify why this is not OK or verify that it is?
That would be potrá nuotare. You can know how to swim and yet not be able to. Illness, bad weather, lack of a suit, etc.
But in this case "saper nuotare" means "essere capace di nuotare", so "to be able to swim" should be correct, i think.
“will be able to swim” could also be “sarà in grado di nuotare”
And how do we know he or she?
You don't. It could be either with no added context.
Sapere can mean to know, know how, understand, be able, but also to find out or to learn. I think a good translation here would be “She will learn how to swim.” But it is rejected by DL.
What's wrong with (he will learn to swim..) ? nothing is known of the subject ! is it a she or he? so?
...dopo lo/la spingeremo in acqua!