thank you Dmitry for explanation of 'had done".
I just want to say some words more.
if the context makes it clear that one action precede or finished before another action, we dont use past perfect but use 2 simple past tenses.
he was born speaking Vietnamese, but he mastered english.
but if context is unclear in that context could offer 2 meanings- two actions happen simultaneously and two actions are in sequence-, the we have to past perfect to show a specific meaning. if we dont use past perfect, the second meaning happens.
from this point, the effective way to test whether past perfect is correct is to replace past perfect with simple past. if this replacement change meaning, the past perfect is correct.
when you came, I had had my lunch
when you came, I had my lunch
so, we have the hard situation. for the same meaning- one action finished before the other-, we still have to use 2 patterns: simple past+simple past and past perfect+simple past.
when you came, I had had my lunch
he was born speaking vietnamese, but he mastered english.
both sentence show that one action precedes the other, but one sentence use past perfect+ simple past and the other uses simple past+simple past.
in other words, there are 2 ways to show one action precedes the other
1. clear context
2. had done
we dont use past perfect when
1. sequence /context is clear
2, we dont care which action come first.
we use past perfect when
we want to emphasize sequence and context is unclear
pls, comment on my explanation. I could come back to revise it