It's under the table. It's beneath the table. Please sign below the line. Beneath means the same as under but it is a formal literary word. In spoken situations "under" will do the job fine. Below means in a lower position, but not covered by something.

to avoid confusion in daily speaking we use under and beneath for being in a position with something above. we use below to describe a position mostly on documents. "please sign below the (dotted) line" it is never please sign under/beneath the line