It depends on what subject you are doing.
Like for example a fantasy
your true life like in the past
my passport,which had expired 3 months ago is new.
Those soldiers were recruited two weeks ago.
Pangaea was a large super-continent that existed millions of years ago.
We were where we should have been, but you somehow missed us. The snacks were on the table a moment ago...
Four days ago I used my kerchief to shield dust from my eyes in the desert.
No.
Three years AGO i went to Hawaii
The sentence is correct if you and the article 'a': "She started her journey a long time ago."
Four years ago, I started this job.
No. It is correct to say, "He received the letter two days ago."
Few years ago, Valentine's day was on a black Friday, and couples broke up on that day. (Very ironical.)
Trains is the subject of that sentence.