"The spoon is under the chair."

Translation: Το κουτάλι είναι κάτω από την καρέκλα.

[kanakakisermis]

"απ'την" must also be added.
By the way. It says I should not report mistakes here. Where am I supposed to report them?

[D_..]

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.

[Lesdio]

Isn't υπό a synonym of κάτω από?

[kanakakisermis]

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.

[Lesdio]

ok thanks