Could you explain the constructure of this sentence?
I am learning French, and one of my exercises below makes me confuse. That is:
On n'a pas retrouvé les chaussures de la victime. (Please change the sentence into the passive constructure)
The answer is "Les chaussures de la victime n'ont pas été retrouvées".
Question: why the word "retrouvées" has the letter "e" at the end? The subject is "Les chaussures", that is plural, not the female, so I think it should be retrouvés.

Question

Answer

On n'a pas retrouvé les chaussures de la victime. 
"Les chaussures de la victime n'ont pas été retrouvées".

Question: why the word "retrouvées" has the letter "e" at the end? The subject is "Les chaussures"

On ne fait pas l'accord en genre et en nombre dans la première phrase parce qu'on utilise l'auxiliaire avoir e le COD vient après l'auxiliaire. Dans la 2ème phrase, pourtant, le passif est une forme composée qui utilise être comme auxiliaire, donc on doit faire l'accord avec "les chaussures" au féminin (+e) et au pluriel (+s)

Answer

In French, 
"chaussures"
is feminine plural. Most (probably all) French words ending in -ure are feminine.
Although French uses the same definite article for both genders in the plural, the words still have their original gender.

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