LOREN: Ist Freitag!
SCOTT: Ist Freitag. Danke... erm... Gott ... ist Freitag?
LOREN: Gott sei Dank. Germans say it all the time, for everything.
SCOTT: What is it again?
LOREN: Gott sei Dank.
SCOTT: Is that Mao sei Dank's brother?
LOREN: Hyuck, hyuck. No, it's a form of "sein."
SCOTT: What is it? Subjunctive?
LOREN: No, it's the imperative.
[Ten minutes later]
SCOTT: The Internet says it's the subjunctive.
SCOTT: But I only looked for websites that would prove me right, so don't put too much store in that.*
LEIGH ANN: Scott, settle a debate for us.
SCOTT: Christ, what did I walk into?
LEIGH ANN: So, if you finish your meal, and you go up to the buffet to get more, do you call that "a second helping"?
SCOTT: No. I would call that... "seconds". When I visualize helping, it's more like, somebody's giving out mashed potatoes, and they plop a scoop on my plate. If I ask for a second helping, I expect another scoop.
LOREN: So it's limited to one occurrence.
SCOTT: No, it's limited to one food, and not replenishing an entirely empty plate.
LEIGH ANN: Well, what about with casseroles?
SCOTT: As, I'm sure you know... as with most situations involving casseroles, all bets are off.
* Compare the English phrase "God bless the Queen." This is the subjunctive. The phrase is a fossil of a time with a more robust use of the English subjunctive. Today we'd probably render it as "May God bless the Queen". Compare this to the indicative form, which would be "God blesses the Queen". The subjunctive implies a wish or uncertainty, while the indicative gives us fact.
Note that the verb in the subjunctive -- bless -- is identical to the imperative form -- e.g., Steve! Bless that donkey! -- and the bare infinitive -- e.g., He helped Steve bless the donkey. Because of this, and because the subjunctive is less common nowadays, some are tempted to reanalyze phrases with it as imperatives, i.e., "Hey you, God! Bless the Queen!" But now you and I know it's not a command at all, and it's a good thing, because you really should not be giving the Almighty orders.
We have something similar with the German phrase Gott sei Dank. "sei" is indeed the second person singular (informal) imperative, but (as in English) it is also the third (and first) person singular subjunctive (I). But, even without getting into the cases of the nouns (which I know little about), you can see that the only real option is the third person singular subjunctive, which would translate as "Thanks be to God." The others -- second person imperative: "Hey you! Be thanks to God! Be God to thanks!" and first person singular subjunctive "I be thanks to God! I be God to thanks!" -- just don't work. See also the Spanish que Dios te bendiga, or the Latin sit dīs grātia or dī tē ament, examples of subjunctive all.
By the way, despite Somerset Maugham's famous quip, the subjunctive remains alive and well. I throw it into our publications whenever I can, and so be it forever.
I look forward to Sasha's prompt corrections of this post.