So a German colleague of mine - someone I know but haven't seen for a while, and have never spoken German with - is visiting us, and popped his head into the office this morning to say hi.

"Hallo [MathNut]!"
"[Colleague]!"
"Wie geht's?"
and I responded: "gut, und dir? Wann bist du da gekommen?"

For an instant I thought he looked a bit startled. I don't know whether it was:
- because I said "du" to him (I'm a PhD student, he's a youngish professor)
- because I said "da" instead of "hier" and it took him a split-second to parse
- because my German is much improved (which it is; last time we met I had basically none.)

The "du" thing was a split-second decision. We're on a first-name basis in English; he's a close colleague of my advisor so I feel like I know him pretty well (socially and academically) from conferences and what not. He seems fairly relaxed and outgoing. On the other hand, he is German (not Swiss), a full professor, and about 20 years older than me, and he did look a bit startled (though like I said, it could plausibly have been for other reasons.)

So now I have no idea what to do next. We've had a couple of brief exchanges since then, but they've been in English. Do I apologize, can I just revert to "Sie" next time we speak German and pretend it never happened, or what? Or possibly "du" was just fine with him and switching to "Sie" now would be impolite. How do I find out?

- MathNut the pronominally inept

Most young Germans are not too hung up about Sie or Du so I wouldn't worry about it. If you're concerned why not simply ask him?

Maybe it's about the time to invite him for Apero and refer to each other on per ''you'' basis

Switch back to Sie in your next German conversation without further mentioning today's conversation, I'd say.

I'm Swiss even the culture to germany is different maybe I can help. Don't worry about this mistake. We normally use Sie but I can not imagine that anyone could get this wrong if you are not a native speaker. And even in our company the english people started to say Du and now it's almost everywhere very fast a Du.
We do mistakes in your English and so do you in german. Don't worry about that and next time you just use the Sie and give him a smile maybe he gives you immediatly the permission to say Du to him or he understands that you just have learned something new. But relax about that this is not that bad!

Did he say Wie gehts or Wie geht es Ihnen?

I was under the impression that "Wie Gehts" is informal and therefore the du would not be a problem.

Wie gehts is also used for people in the Sie form. But you would not write that in a letter but verbal it's ok with SIE

If this is the case then I have made a similar mistake many, many , many times. Even my Swiss girlfriend uses this rule of thumb!

wie geht's oder wie geht es is just the short form of wie geht es ihnen oder wie geht es Dir it has nothing to say if you talk in Du or Sie with this person. Just the ending Dir oder Ihnen let you know it

simple : ask him/she

I knew a friend who made such amistake 3 times and then no one ever heard of him! Life can turn miserable some times!

got executed and buried ??

I just passed the question around the office.

Consensus from my Swiss work mates is that yes "Wie Gehts?" can be formal, but usually it is not.

They also reckon that because you are first name terms in English and there is a history, there should be nothing to worry about.

What I do when I am unsure is try to avoid du/Sie or directly addressing the person until I here how they address me then I use what they use.

Its bloody confusing though, My gilrfriend always told me that in a sports shop I should be per du, but lately I have noticed more people speaking per Sie with me in this situation. Maybe I'm just getting old!

Made to eat fondue for breakfast for 30 days

maybe you are

I would imagine he just got a surprise that you spoke to him in German, and so spontaneously... Nobody with a head on their shoulders could possibly take offence at such a mistake from an English speaker when they know English themselves and that this form doesn't exist in english. But all the same to avoid the risk to those who may not know, try to remember in future. As you are younger surely that means that even though they use du to you, it is still polite to use Sie for them?

just remembered how many really stupid things I said in English when I had to act in such a situation (spontanous) at office

The other clue here which I have also been told is that Hallo is informal. Gruezi is what is used as a formal Hello in Switzerland. I constantly get into trouble with people over this one. I keep forgetting who I am supposed to Hallo and who I am supposed to Gruezi! (Edit: Zurich area. In other places Zurchers are called Gruezers because of how they greet people!)

Of course this is all different in High German so I believe!

For me difficult is still if somebody asks you how do you do? Or how are you then sometimes an answer like "I'm fine thank you and you" is not excpected. That is quite strange

Many thanks for all the advice and consolation. Macchiato, this is the first time I've ever done it (to my knowledge!) so I think I'll be OK... treading on thin ice though...

Yeah, normally if I'm unsure with someone I'm very good about listening to hear what pronoun they use, but this time there was simply nothing to go on.

I'm supposed to meet with him tomorrow afternoon - that'll be in English - maybe if I get an opening I'll just ask him then. In the meantime I guess I'll avoid pronouns if at all possible, and use "Sie" if I must use something.

Question: if I ask him and he does say he would prefer "Sie", should that also be my cue not to call him by his first name any more (in German)? It's always seemed to me that there's some correlation there... but maybe I'm just perceiving a rule because I so want there to be one.