Example: I met this person once at a meeting a year ago, today I am meeting with him again.
Should I say:
We have met a year ago.
We had met a year ago.
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
When you are describing something that took place in past and you need to say you had met someone before that event in past then then you shall use 'had met'.
For example, suppose you are describing a traffic stop to a friend. You might say
The cop stopped me. He asked me for papers. I remembered I had met him before in a shop.
In other cases you should use 'have met' or other variations as suggested by commenters.
This is a typical scenario describing an action started and completed in the past, as the adverbial "a year ago" clearly specifies. The obvious candidate is the simple past: "We met a year ago". If you, as you ask in your comment, want to stress that the action took place several times, just add a frequency adverbial: "We met multiple times a year ago".
'I have met Mrs. Smith' is a present tense. (Known as Perfect, or Present Perfect.) It is used of a current state of affairs related to the past action or actions expressed in that tense. Here, it carries the connotation: 'I am acquainted with Mrs. Smith, (because we met in the past)'.
'I had met Mrs. Smith' is a past tense. (Past Perfect, or Pluperfect.) It is used of a state of affairs in the past, related to the action or actions before that. For example: 'I had met Mrs. Smith before she gave me a job interview'
'I met Mrs. Smith' (Past Simple) is used straightforwardly of an action in the past: 'I met Mrs Smith on July 12th last year'
'We have met a year ago' ...
..is actually a little awkward. It jams the past and present into the same phrase, and could be perceived as a mistaken use of the Present Perfect, where the Past Simple would be appropriate.
It almost asks for a comma: 'We have met, a year ago', which makes the adverbial phrase parenthetic, and could be more clearly interpreted as a shortened version of: 'We have met. (We met a year ago.)'