arigatou: gozaimasu vs gozaimashita

[Yizzo]

haha 最近たくさん日本語 のもんだいがある!すまない!
lol i dont even know if what im typing makes sense or not でも, 日本語を練習したい。
違うことがあるので、feel free to correct me!

いつ「ありがとうございます」と 言う? (was that the right particle?)
いつ「ありがとうございました」と 言う?

and are they interchangable? eg. in a restaurant, when you're about to leave, should the waiter say thank you in past tense?

ありがとう 、

Yizzo

[Garyuchin]

ありがとうございました　after a transaction is completed
ありがとうございます　during a transaction.

[Bueller_007]

Already covered in detail elsewhere on the forum.
http://www.japanesepod101.com/forum/viewtopic.php?t=721

The answer to your question is that the waiter will normally use past tense, but present tense is also acceptable, and may have a nuance of "thank you not only for coming this time, but thank you for your continued patronage in the future as well". But if you get "gozaimashita" instead of "gozaimasu", it certainly doesn't mean that the waiter is dissing you or implying "don't come back again", etc. For all intents and purposes, they're identical in your example.