Spanish - prepositions, sitting, and tables

So, first on the list here is becoming fluent in spanish by, uh, July 15th or so. I'm not starting from scratch, as I took three years of spanish before in school, but it's been a while since I tried to really speak it, and low blood flow to the brain plus over a decade has knocked most of it into the shadowiest corners of my mind.

But I have the internet, I have Rosetta stone (which actually helps me, personally, as long as I have supplemental materials), and I have the motivation to get better at this so I can be of more use to my kids as THEY are learning spanish.

So, here's what I start tracking down as I'm going through Rosetta stone, to supplement what I'm learning/re-learning on it.


Two phrases that are very similar:

El niño está sentado en la mesa. Means: the boy is sitting on the table. 

and

El niño está sentado a la mesa. Means: the boy is sitting at the table.

Important distinction. :-)