nuestras vs nuestros
Does the choice of these two words depend on whether or not the noun that follows is masculine or feminine or on whether "our" is referring to men or women? I'm not sure why it is incorrect to translate "They read our letters" to "Ellas leen nuestros cartas." The correct answer given used the word "nuestras."
The possessive words are like adjectives; their gender comes from the noun they describe. In your example, the noun being possessed is "cartas" (feminine), so the possessive must be feminine too, "nuestras".
Thank you for this. I was wondering why "Nuestros caballos beben leche" was correct, and "Nuestras caballos beben leche." wasn't. The pronoun takes the gender of the noun it's describing. ^^
The gender and number will always reflect the thing or things possessed. All the possessive pronouns change to reflect the number of things possessed (i.e. tu/tus), but nuestro and vuestro also change to match the gender of the thing or things as well.