"This child is you."
Translation:Dieses Kind bist du.
This got set as part of the formation of the English language. It could have gone either way, but now that this way was picked these are two different right answers, one for each language. Fluent English always matches the 1nd/2nd/3rd person of preceding noun to conjugate the copula, but fluent German always picks the lowest numbered person in play, regardless of word order.
3 April 2018 - There is no problem with it having the copula. The problem is that the sentence could have either das Kind or du as the subject, each requiring a different conjugation.
I shall report this, as the English sentence is quite clear that the subject is this child/dieses Kind . This child is you is a different sentence from You are this child. Not a big difference in meaning, but the translation is, IMO, valid.
I'd say the issue comes with which form is the most natural one in the language at play. For instance, imagine you and your friend are looking at an old album of photos and point at one photo of hers in particular, then in English you would say "This child (in the photo) is you" using the verb in 3rd person referring to "this child", however in Spanish you would say "Esta niña (de la foto) eres tú" using the verb in 2nd person referring to "you". It's just the way it is in the natural native expression. I guess the same happens with German, it must be more natural to express this idea as "dieses Kind bist du" instead of "dieses Kind ist du".
That means, if you were to translate "dieses Kind bist du" into English, you would sound more natural by saying "This child is you", not "this child are you".
I think the English literal equivalent (and not the idiomatic translation) to "dieses Kind bist du" would not be "this child are you" but "you are this child" since in German the subject can naturally appear after the verb, or so I believe. "You are this child" still sounds a bit weird, but not as much as "This child are you".
I think it is a special case of English, because I made the translation to Spanish and made sense: Este niño eres tú (To be conjugation of "You") Sounds correct. Este niño es tú (To be conjugation of "he/she/it) sounds wrong. Anyway, It would be better to state the sentence: You are this child.
In English we treat the second kind of sentence as having a subject and object, with the word order changing which is which, but in German they are stricter about the verb 'to be' not having an object (both sides are treated as the same thing, the subject of the sentence) and changing word order only changes emphasis. So, "This child is you" becomes "Dieses Kind bist du".
When you have a sentence in either English or German using a linking verb (copula), both nouns (or the subject noun and the predicate adjective) are in nominative case. Jack is a nurse/ Johann ist eine Krankenschwester. Dich is accusative (direct object) form of du, so it isn't used in a copula.
In the English sentence you provided, the subject is "this child" (Dieses Kind), the verb is "is" (ist), and the predicate nominative is "you" (du). If you wanted students to respond with "Dieses kind bist du," then the English equivalent would be something like "This child, you are." Or, if German word order for this type of expression is frequently different from English word order, then it would be helpful if this is explained to students. Thanks.