"I do not want your soup to become cold."
Translation:Non voglio che la tua zuppa diventi fredda.
"Diventi" isn't about you (or tu). It's a subjunctive form. Here's a reference: http://italian.about.com/od/verbs/a/italian-verbs-present-subjunctive-tense.htm
"Non voglio che la tua zuppa raffredda" that's what i wrote. but it didn't accept it. Why is "si" necessary. The soup just cools (down) but it doesn't do that actively. Also why raffreddi instead of raffredda? It cools (down), is 3rd Person singular. Or not? Sorry, that sentence came a bit too soon if you ask me. Edit: just looked up congiuntivo in Wikipedia from what i understand it's because the "che". So way to early for this sentence. btw i wrote "diventa" in the corrected sentence and it accepted it. so this actually needs to be congiuntivo too
If you say just "raffreddi", that means that the soup cools something else actively. ("raffreddare" is a transitive verb".) "si raffreddi" means that it cools (itself) or becomes cool.
You need the subjunctive form "si raffreddi" rather than "si raffredda" because of "voglio" in the main clause. This may help: http://ciaoitaliablog.wordpress.com/classes/the-subjunctive-mood/
Hallo gulli... I'm afraid my English is not good enough to explain this. As I presume you speak German, let me use my mothertongue: In romanischen Sprachen wird für mit Wünschen, Denken, Hoffen, Befürchten und Gefühle ausdrückenden Verben für das Verb, das Gegenstand dieses Wünschens o.ä. ist, der Subjonctif (im Französischen) benutzt. Beispiel: Ich weiss dass Du tapfer bist - Je sais tu es brave. Aber: Ich will dass Du tapfer bist. - Je veux que tu sois brave. Im Italienischen ist das genauso. (Nur: Wir hatten dieses Grammatik-Thema hier noch nicht.)