Babies should not sleep on their tummies. They should always sleep on their backs.
From what I understand, body parts usually take on the article , "the". ie. "le", "la" and "au". You assume it is the subjects body parts when in discussion.
This is shown in many examples in duolingo: ex. "Il a le visage rouge" = He has a red face. His face is red. ex. "J'ai mal a l'epaule" = My shoulder hurts. I have a sore shoulder.
I assume this has something to do with the literal translation, "J'ai mal a l'epaule" being, "I have a pain to the shoulder".
If I am incorrect or anyone else has something to add, please do.
Why is "the baby sleeps on the tummy" marked as wrong? If it is wrong, what would be the correct French translation if I wanted to say "the baby sleeps on the tummy"?
the doesn't work here. If you know the sex of the baby you can use his or her. If you don't you can use its. We used to use his with the indefinite person, but it's dated and considered sexist. We don't know the context so his, her_and _its all seem fine to me.
The word "tummy" implies a cute, sweet abdomen, like when you tickle a toddler. Does "ventre" also imply cuteness? It sounds very anatomical to me.
If the baby is sleeping on his father's tummy then too the structure will be same. The baby is with his father. The baby is sleeping on his tummy.
The baby could also be sleeping on somebody else's tummy. You can't tell from this one sentence. In that case 'the', 'his' and 'her' are equally right.