"The spoon is under the chair."
Translation:Το κουτάλι είναι κάτω από την καρέκλα.
"απ'την" must also be added. By the way. It says I should not report mistakes here. Where am I supposed to report them?
You can report mistakes through the report button on the website and the Android app as far as I know. Otherwise here in the sentence discussions, it's fine.
Almost. υπό is a quite old fashioned way to say "under" except for some standard phrases. You could use it but then the correct sentence would be: "Το κουτάλι είναι υπό της καρεκλός". It is correct but people would think you are a time traveler.