The role of "sein" with a verb in "infinitive"

Question

I’ve recently come across a sentence:

Einmal war ich nach einer Party um zwei Uhr morgens Döner essen mit Jamaal, einem syrischen Freund.

What I didn’t understand is what war is doing here? When I translated the sentence with Google Translate, its translation was:

Once, after a party at two o’clock in the morning, I was eating kebabs with Jamaal, a Syrian friend.

So is the verb to be sometimes used to create a continuous form in a sentence? But I was under the impression that it’s not necessary; we just use the simple tense to make a continuous sentence.

Answer 1

German knows 3 general possibilities of expressing the English continuous tense:

• by adding temporal adjective markers, e.g. "ich esse gerade"
• by using sein + am/beim + Infinitiv (aka Verlaufsform, probably the closest option to the English continous tense)
  • by using sein + Infinitiv (aka Absentive (wiki:en)), which also happens to be your case.

The latter possibility is often used when expressing a state of being busy, often with a local subtone to it (not being able to "be there"), e.g.

-Wo warst du denn? -Ich war einkaufen.

An example of the second possibility: (nobody's absent or busy, just expressing a state)

Mann, ich bin am Verhungern! - Hey, I'm starving!

It is debated whether there the 3d possibility deserves the name "absentive", but we can still use this as a shorcut for understanding this complicated business.

To your question, if it's necessary or not: unlike with English, you can perfectly do without these constructions, they are used predominantly in colloquial speech and can be painlessly substituted by saying e.g. "Ich kaufe gerade ein, in einer Stunde bin ich wieder da".