"Good day, we are beginning!"
Translation:Guten Tag, wir beginnen!
So, when a sentence begins with anything other than the subject, the verb comes before the subject. I knew that things as simple as "Ja, ..." and "Nein, ..." were exceptions to this, but apparently "Guten Tag, ..." Is an exception as well. Is the rule simply that if the term at the beginning is not grammatically related to the sentence, then it's okay?
The term "Guten Tag" is separated by a coma, so the real sentence structure begins with "wir" the subject. The same with names: Tom, ich sehe dich. or Hey Tom, ich gehe schwimmen.
(More difficult are structures like: Obwohl er groß ist, ist er ein Kind. ~ the subordinated clause "obwohl er groß ist" forms the first position and the second "ist" is the finit verb at the second position.)
I'm not confident, but I'm going to give this a shot. It's an adjective, so gets an appropriate ending, and since there is no article, the ending matches the "der/ die/ das" forms, except for reasons I am not clear on, this is accusative, so the ending is "-en" for this masculine noun. If I am wrong, someone please clarify so I don't just confuse the issue.
One of the English translations proposed here says "Good day, we're beginning". Not sure how the other English speakers feel about this, but to me it sounds unnatural to use "good day" as a "hello" rather than a "goodbye" greeting… more like too literal a translation of the German phrase here, perhaps.