Hi, nur ganz kurz der Hinweis auf die Foren, die dieses Thema schon dutzendfach durchgekaut haben. Suche mal nach "relative clauses", "Relativsatz / Komma", "defining clauses", non-defining clauses" etc.

" defining relative clauses
Defining relative clauses identify the noun being referred to. They tell us which person or thing is meant. They follow immediately after the nouns that they modify with no pause in speech, so no comma is used in writing:

The boy (that) I dated when I was at school has now married my best friend.

The shares (that) he bought when he joined the company are now worthless.
Note that if we leave the identifying relative clauses out, the sentences do not have very much meaning:

The boy has now married my best friend. (Which boy?)

The shares are now worthless. (Which shares?)



non-defining relative clauses 
Non-defining relative clauses do not identity what is being talked about. They simply add information about the person or thing already identified. Non-defining relative clauses are normally marked by pauses or intonation breaks and therefore also by commas:

My friend Deborah, who was my closest friend in primary school, has now married my cousin.

Trevor Nunn's production of 'Anything Goes', which opened at the National only two weeks ago, has nearly sold out.
Note that if we take these non-defining relative clauses out, the sentences still make perfect sense provided the context is known:

My friend Deborah has now married my cousin.

Trevor Nunn's production of 'Anything Goes' has nearly sold out."

http://www.bbc.co.uk/worldservice/learningeng...

Ich würde keine Kommas setzen.

When the antecedent has an indefinite article, as here, it is difficult to distinguish between 'defining' and 'non-defining'. In fact I'm not sure whether the distinction has any content at all in such circumstances. Your comma, then, must be governed by other (subjective) criteria. Do you see the relative clause as a sort of parenthesis or afterthought -- if so, use commas. If not, don't.

@Braunbärin: Dass es im Forum sowie auch im Interent die üblichen Erklärungen zu diesem Thema gibt, war mir klar. Reinhard W. war auch so nett diese sogar aufzulisten. Jedoch beinhalten diese Regelangaben nie einen konkreten Hinweis darauf, was bei meinem konkreten Beispiel zu tun ist, bei welchem der "antecedent", wie er von escoville genannt wird, mit einem "indefinite article" beginnt, und wenn ich dafür nach einer Regel suche, dann suche ich nach einer Nadel im Heuhafen.

@Escoville: Thanks for your explanation. Maybe the title I used for my post was ambiguous since it seemed to have been misunderstood by some people who thought that I wanted to know the general rules. Indeed, it was the indefinite article right at the beginning of my example sentences that confused me, and you understood what I tried to point at. I also thought that probably both versions - with or without the commmas - might be possible. Thanks for your reply.

Die non-defining relative clauses, die durch ein Komma abgetrennt werden, lassen sich durch einen kleinen gedanklichen Zusatz erkennen:
My friend Deborah, who was my closest friend in primary school, has now married my cousin.
(Meine Freundin Deborah, die - übrigens/nebenbei bemerkt - in der Grundschule meine beste Freundin war, hat jetzt meinen Cousin geheiratet.)

Für dein Beispiel:
A man, whom I had not met before, talked to me yesterday.
(Ein Mann, den ich - übrigens/nebenbei bemerkt - vorher nie getroffen hatte, redete gestern mit mir.)

Meiner Meinung nach kann nach dem einleitenden "a man" (also unbestimmter Artikel am Anfang) kein notwendiger ( defining) Relativsatz folgen, d.h. hier sollten Kommata stehen.

Die beiden Arten von Relativsätzen lassen sich formal nicht unterscheiden, aber die Zugehörigkeit zu den defining relative clauses läßt sich durch die Möglichkeit nachweisen, ein spezifizierendes "und zwar der-, die-, dasjenige" einzufügen.
Das ist nach einem unbestimmten Artikel nicht möglich.

For your example, no commas.

While Reinhard's (#6) logic looks good, his conclusion is wrong. In part this is because he is relying on the names "definite article" and "indefinite article" rather than the uses. An indefinite article can certainly be specific. Consider for example the following palindrome: "A man, a plan, a canal, Panama". We have a specific man, plan and canal even though the phrase uses the indefinite article. In English, words with indefinite articles can have defining relative clauses attached. (A hero is a man who puts others before himself.)

I am with hm -- us and Gibson in saying that the most natural-sounding sentence is the one without the commas. At the same time, escoville's comments about the subjective understanding (afterthought or not) are germane.

Thanks for all your comments!