"¿Sabes nadar bien?"
Translation:Do you know how to swim well?
Exactly. But isn't this "correct solution" word-for-word? And do they mean you know well or you know how to swim well? What does the word well define: the quality of the knowledge or the quality of swimming? The English version seems to imply the last but the the Spanish sentence seems to imply the first meaning. That's what's confusing. I'd appreciate a Spanish speaker to clarify.
This is a great question. I spoke to my husband (from Honduras) about this, and it took a while for him to understand my question. Hahaha. But ultimately, he said to express the quality of knowing, you would say "Sabes bien como nadar." But he also said this would be more of a statement than a question.
The correct solutions should not be word-for-word translations, but the best translation of the meaning of a statement. Ideally that is what Duo tries to do, although they of course make mistakes. In this instance, "saber" followed by another verb, means "to know how to" not just "to know". Sunflower64420 is right. It's not about literal translation, but about understanding.