I see it as an action question verses a to like question. "Since when does he drink tea" is asking when all of the sudden does he like drinking tea and "Since when has he been drinking tea" to me means when in time did he start drinking the tea he is currently drinking
Adam1978's translation is fine and should have been accepted. "Seit wann trinkt er Tee?" Can express surprise that he likes tea, in which case the English equivalent is "Since when does he drink tea?!" but it can also mean a straightforward question, for example in the case of a small child who you notice drinking tea for the first time, "Since when has he been drinking tea?" Unlike English, German uses the present tense in a case like this, to express a situation that is continuing into the present and is expected to continue into the future. Another example: "Seit wann wohnst du hier?" = "How long have you been living here?" (Also, KendallHolm, the past participle of "trinken" is "hat getrunken.")
Sounds ok. Also Janet1930 - I agree - "When did he start drinking tea" would be the preferred expression for this Australian too. "When did he start tea-drinking" might be even less ambiguous, or even "When did he take up tea-drinking".
Or "How long has he been drinking tea?". It's ambiguous, but maybe the German sentence is also ambiguous.
OR How about "How long has he been a tea-drinker?"
How long has he drunk tea? How long has he been drinking tea? Since when has he been drinking/drunk tea? The English translation Since when does he drink tea? (to my UK ears) sounds as if he has only just started drinking tea and the speaker is expressing surprise. I don't think that is what the German means.
This one always gives me a double take, in my english (SSE) I would never say 'since when does' I would always treat it as the whole sentence was past tense, I would say 'Since when did', because 'since when' is a question about the past. Clearly the purpose of the question is to illustrate that this is how they say 'since when did' in german, but in german it's not treated as past tense.
Seit wann trinkt er Tee. The translation ' since when does he drink tea' is very un English if you are asking when he acquired the habit. This sounds more like a statement of disbelief and would probably be expressed as ' Since when did HE drink tea'?. Admittedly not great grammar but we all have our quirks
None of it, really. There is confusion in a couple of the comments above that refer to the dative case. "Seit" takes dative only when functioning as a preposition followed by a noun and its modifiers. Examples: "seit unserem letzten Treffen" (since our last meeting); "seit ihrem Geburtstag" (since her birthday). But when followed by a time adverb like "gestern" the dative case is irrelevant because adverbs aren't declined. "Wann" is similarly an undeclined adverb. Also, "seit" can function as a relative adverb, introducing a whole time-expression clause, just as we use "since" in English. In that case, the clause that follows it begins with a noun in the nominative. Example: "Ich habe nichts gesagt, seit meine Mutter nach Hause gekommen ist." ("I haven't said anything since my mother came home.")
I agree with everyone else who has said "since when does..."isn't typical English as "since" is typically followed by the past tense.
I note one person said "since when has he drunk tea?" was accepted, in which case "since when did he drink tea?" should also be. They mean pretty much the same, with the second formulation perhaps indicating a greater degree of surprise.
your example CapnDoug is stilted.....an English speaker wouldn´t use ´since´ at all but would say ´how long has he been drinking tea´ because German uses the present tense after seit and if we use since in English it is followed by the present perfect or present perfect continuous.