"They are not a couple."
Translation:Αυτοί δεν είναι ζευγάρι.
Since the indefinite article also means 'one' it is often dropped. They could not be anything other than 'ένα ζυγάρι' - 'one couple', so the indefinite article is implied in the phrase.
I was just about to post the same question that mcampanella did. I have read your response, and I understand the implication of ζευγάρι, but is it indeed incorrect to include the indefinite article? Also, what about when it means "pair," as in "I would like a pair of shoes" -would that be Θα ήθελα ένα ζευγάρι παπούτσια or no? And what if there is an adjective, as in "I would like a new pair of shoes" -is it Θα ήθελα ένα καινούριο ζευγάρι παπούτσια?
In the sentence above it can be dropped because, we two cannot be two couples. In the second sentence the indefinite article is necessary because I could want two pairs of shoes. As a rule of thumb, where you can have no more than one, the indefinite article can be dropped.
I wish there were a clear rule about when to use the indefinite article and when not - as a native speaker I can only say what sounds natural but not why it is so!
If you add ένα in the sentence given above, it will sound like 'one', not 'a'. (And it works the same way in the affirmative.) Could it be for the same reason why 'a' is not included when you replace 'couple' with 'friends'? Or 'colleagues' or something similar...
For your examples with ζευγάρι=pair, including ένα is actually required in both cases.
It seems that there is no clear rule, as I know. But you can think when should or shouldn't. The indefinite article in Greek is the same with the arithmetic ένας, μία (μια), ένα. In the above case there is no need to show the number, so should we include an indefinite article? Look: Αυτοί δεν είναι ένα ζευγάρι. It is redundant an article here. Unless we want to show that this couple is a whatever couple, but a special couple, so the emphasis goes to ένα, even with stress in voice to ένα, or according to the context in a written text.
I hope it helps.
Generally speaking this is the case that the indefinite article is important to use in Greek. To show that someone or something are whoever or whatever.