It's kind of difficult even for native speakers. Some people always use the present tense version no matter what. Other people use the past tense version for when more than a certain period of time has passed since someone did something to you, and the present tense thanks for when it's on going, just happened or happened very recently. There doesn't seem to be a clear cut line when it comes to when to use which, and different kind of thing requires a different amount time to pass to use the past tense version. This seems to be cultural, personal, and possibly regional to an extent.

It seems people who always use the present tense version are often determined to do so for some reason. Using the past tense thanks when it sounds strange could be frowned upon by those stubborn people while using the present tense version rarely causes a problem. You could say some goblins claim you're not thankful now if you use the past tense thanks; they take it as "I was thankful. But now I am not." I don't think those curmudgeons would get upset by non-native speakers, though.

I could be wrong on this one, but I think most Japanese would say ありがとうございます when referring to something that just happened, and a lot of people say so too if it's in the same day.

EDIT: I found an explanation in Japanese:

http://home.alc.co.jp/db/owa/jpn_npa?stage=2&sn=189

It says the past tense version is preferred when it is used with a word(s) that usually appears in a past tense sentence such as 先日, 先ほど, and 先だって. Other than that, my explanation more or less agrees with it. Also, the phrases are sometimes indistinguishable from a greeting like a shop clerk's ありがとうございました , so the rules don't always work.

Last edited by magamo (2009 November 08, 2:22 pm)

Wow I didn't know it was so complex :p
I guess I'll just stick with ありがとうございます except in situations where the other would clearly be appropriate.