How exactly is "He had written a letter" different from saying "He wrote a letter"? Both suggest past events that have concluded already.
EDIT: nevermind, n6zs puts it best: "The whole point of the pluperfect tense is to express an action which happened in the past before another past action."
Past perfect is "past in the past".
He had written his letter before I wrote mine.
The verb tenses in that sentence are non-negotiable.
Should there be a liaison here? The audio says "il avaiT écrit" pronouncing the T in avait. It doesn't sounds right to me, but I'm not a native speaker and I can't explain why.
What I can say is that to a French ear, the phrase without a liaison sounds a bit ugly. "ai" followed by "é" doesn't feel very natural, it flows better with the liaison.
Facultative liaison. See (in French): http://fr.wikipedia.org/wiki/Liaison_en_fran%C3%A7ais#Liaison_facultative
The liaison can be used optionally, but when using liaison you have to connect the two words. On the audio she pronounces the "t"' on "avait" and then pronounced "écrit." That is incorrect. If using liaison, the correct way is to pronounce the last letter of the word as the first syllable of the next. It should be pronounced "técrit" if using liaison.
If the liaison continues in this sentence between .. écrit une .. , as asked by Konrad-Michal above, might you hear the whole as "I lavai técri tune lettre" or as "Ilavaitécritune lettre" or with optional liasions where chosen?
"He had wrote" and "He had written" mean the same thing. One version is US English and the other is British English. This needs to be fixed, all to often the US English answers are favoured. I am not here to learn US English and this is not a negative comment that I am writing, it is just how it is worded in British English.