From what I can tell, saying "en la mesa" could mean that you put them on or in or at the table, whereas if you specify "sobre la mesa" then you put them on top of the table. It's just more specific.
Then again, I'm learning too. Is anybody going to come along and point out that I was completely wrong?
I see...that sounds like it makes sense. So you're thinking it would be more used at times when we would say "on top of" instead of just "on"? So it would be used in a case like this:
Person #1: "Put the box on the table." (Pone la caja en la mesa.)
But person #2 starts putting the box under the table.
"No put it on top of the table!" (¡No, la pone sobre la mesa!)
I wondered the same thing, knowing that there are irregular verbs in all languages. I read this page, and found the answer near the bottom. I guess time and memorization will fix my issue with the accented "o" and hearing other cues to know who is doing what and when...
Guess what! You got fooled by somebody else who called the list, hints. The pull down list are not hints. They, in most cases, show various other uses of words, and are presented for educational purposes, and not deviously to trick students, nor to serve as a choice.. And a lot more are shown in your Words list and none of them are hints..