Taurus wrote:But how do you know which tense to use when thanking someone for something? I guess you use the past tense to thank someone for something they've done in the past, right? Does that mean that if, say, a store clerk hands you your change, you thank them using gozaimashita? Because, technically you're thanking them for something that they've just done. But it seems to me that in this sort of circumstance, people tend to use gozaimasu. I find it pretty confusing.
Gozaimasu is used for something taking place, JUST DONE or already done but relative things are contineing.
Gozaimasita is used for something completely done.
When someone is keeping a door open for you, you say "Arugatou gozaimsu"
When you just recieve something you dropped form someone who picked up it, you say "Arigatou gozaimasu".
When you just find copies on your desk that you asked someone to reproduce, you say "Arigatou gozaimsu"
※ In this case, you may feel gratitude at the moment you find them even someone did before.
Someone saw you off the other day, you say "arigatou gozaimasita".
You were invited to a pary the other day, you say "arigatou gozaimasita".
You recieve a present the other day, yo say "arigatou gozaimasita".