it's idiomatic, it's there to insist that you should go ahead and not protest out of politeness (which makes it part of the politeness fest itself, really)

The insisting part is the one to remember, only the context makes it polite. I believe it can be used to stress an interdiction too (native speaker to help here?)

Nur = only, just.

It does not sound good with 'nur' at the begin, because it is a command. But 'Nur zu, gehen Sie voraus.' is okay.

"Geht" implies that the subject is "ihr," so a correct translation could be "Geht nur voraus, bitte" if you are talking to multiple people.

I thought the same. Worth reporting, I think.

to start doing something = anfangen/beginnen, etwas zu tun

• Die Ergebnisse sehen gut aus, fangen Sie bitte einfach mit dem nächsten Test an.
• Die Ergebnisse sehen gut aus, beginnen Sie bitte einfach mit dem nächsten Test.

oder "Die Ergebnisse sehen gut aus, fahren Sie fort".

Except during a Duolingo lesson on directions, better translations of "Just go ahead" (which has nothing to do with directions) include "Dann nichts wie los, bitte." and even "Bitte, bitte, bitte."

Duolingo frequently trips over translating English to German. The content contributors apparently don't know English as well as they know German. I understand that the software generates additional exercises, but presumably someone looks at what the software comes up with.

I was wondering the same thing... anyone?

No, the sentence 'Nur gehen Sie voraus, bitte' is not good. 'Nur zu, gehen Sie voraus.' would be good, but is not asked here.

• Nur zu! ~Do it!

it would mean more something like "go ahead for now, please", in the sense "yes, we'll get to what you're saying, but let's start with you going ahead"

I'm part guessing :(

Soeben haben Sie den Knopf betätigt. (past)

Soeben passierte ein Unfall. (past)

Soeben werden wir in eine andere Welt gezogen. (present, passive + scient fiction)

Soeben and active? --> Soeben bin ich aufgewacht. (past)

But 'soeben + active + a commant' - I think it is not possible!

It is nor a common in use nor a valid solution.

It does not tell the order you want to tell.

Was sollte jemand angehen? - Soll jemand seine Alkoholprobleme angehen? Soll jemand seine Aufgaben angehen?

But you can say: Gehen Sie bitte voran?

The grammar is fine, BUT it does not work like you want that it work.

"gehen Sie geradeaus" with and without "nur" mean go without any right hand bend nor left hand bend.

(Like Margusoja tried to explain it, it is not possible to translate very well the word "just" here.)

"nach vorne" indicates a place, it does not indicate a direction.

"Fahren Sie bitte voraus." would be useful in the situation in which you ask somebody to show you the right way by going by car/bus/train/cycle infront of you. Or in the situation that you will arrive late at a location and the other person should already start to go by car/bus/train/cycle to the location without you.

So normally you say "go ahead" in the meaning of "be an idol" and "start now" or "continue". I think "Fahren Sie fort"(=continue) is the best translation for "go ahead" with "fahren" and without context.

The formal form is "Gehen Sie nur voraus, bitte." The "Sie" is a must be.

The informal, singular form is "Geh nur voraus, bitte." (=du-form). An alternative du-form is "Gehe nur voraus, bitte." In an order for "du" the personal pronoun is not used.

The informal, plural form is "Geht nur voraus, bitte." (=ihr-form) In an order for "ihr" the personal pronoun is not used.

I could say I despise people who despise other cultures. But I won't. I only disagree with this opinion. The distinction between formal address and informal is common in many languages: German, French, Spanish, etc. If you want to learn a new language, you need to adopt, at least linguistically, new rules.