"The leisurely women write many letters."
Translation:Feminae otiosae litteras multas scribunt.
You need some explanations about the ADJECTIVE, ōtiōsus, ōtiōsa, ōtiōsum .
The adjective comes in 3 genders (M / F / N) and 2 declensions: the ōtiōsus set of forms are MASCULINE and 2nd declension.
The ōtiōsa set of forms are FEMININE and 1st declension.
The ōtiōsum set of forms are NEUTER and 2nd declension.
If the women are at leisure, they are: fēminae ōtiōsae (nom pl), fēminārum ōtiōsārum (gen pl), fēminīs ōtiōsīs (dat pl), fēminās ōtiōsās (acc pl), fēminīs ōtiōsīs (abl pl).
If you have NEUTER "idle carts," they would be: plaustra ōtiōsa (nom pl), plaustrōrum ōtiōsōrum (gen pl), plaustrīs ōtiōsīs (dat pl), plaustra ōtiōsa (acc pl), plaustrīs ōtiōsīs (abl pl).
If the old men are at leisure: senēs ōtiōsī (nom pl), senum ōtiōsōrum (gen pl), senibus ōtiōsīs (dat pl), senēs ōtiōsōs (acc pl), senibus ōtiōsīs (abl pl).
In the Duo sentence, it's "the women" (Fēminae, nom pl feminine) who are "at leisure," and so we need a feminine plural nominative adjective to describe them: that's ōtiōsae . We could have "mothers at leisure": mātrēs ōtiōsae . (I add this to show that adjectives don't necessarily 'rhyme with' their nouns--if they are of different declensions, they CAN'T have "the same" ending.)
I know it's confusing, but take care not to confuse the -a ending that's feminine singular nominative, and the different -a ending that's neuter plural nomin/accus.
I hope this helps! (If you ask a specific question, I'll do my best to answer it.)