I really appreciate this example being included.
I am wondering if "One minute ago" should be accepted as correct as well.
I have met several Greeks (in Greece) for whom English is a second Language, and they simply do not use the word "Ago", but are always using the word "Before". e.g. "Before one minute,..."
It's a common error and I can imagine you heard it often. Yes, "one" can be substituted for "a/an" most of the time. Thanks for the input.
What you have written is correct but it is not a translation of the Greek which has no reference to "a minute has passed". It simply says Πριν = "ago/before", ένα = a / one, λεπτό = minute. No reference at all to the passing of time.
You need to reference those three words.