So "arigato gozaimasu" is "thank you" and "arigato gozaimashita" is "thanked
you"??? Now you know why this poor ole country boy gave up...
Returning now to my Jeff Foxworthy picture book...
Well "Arigatou" derives from "arigatai", originally meaning "rare, precious", and thus to the modern meaning "grateful, thankful". "Arigatou gozaimasu" is a rather archaic form essentially meaning "(what you do/have done/will do for me) is rare and precious (and thus I am grateful)". "Arigatou gozaimashita" essentially means "(what you did for me) was
rare and precious (and thus I am grateful)".
So basically you use "Arigatou gozaimasu" for something someone is doing or will do, and "Arigatou gozaimashita" for something they already did. Although "Arigatou gozaimasu" can also be used in that situation. My understanding is that the Japanese people collectively decided to do this just to screw with newbies to Japanese.