"Voglio che la tua zuppa non diventi fredda."
shouldn't it be "diventa" since it is the soup that should not become cold?
"Diventi" in this case is 3rd person present subjunctive (congiuntivo); "che" often introduces subjunctive subordinate sentences.
but in the description of error it says: you used the lui/lei form diventa instead tu form diventi. and the soup is definitely not you (tu), so the error description is wrong... and congiuntivo was not covered in the part i'm at so it should not be regarded as error :-)