This sentence should be:
Ustedes mandan cartas a su abuela cada mes.
Translation:
You(formal) send letters to your grandma each month.
Ustedes- you(formal)
Mandan-send
Cartas- letters/cards
A su- to your
Abuela- grandma
Cada- each
Mes- month
The sentence you provided was grammatically incorrect since cortas means to cut and Mrs. is used to introduce a married woman which should have been mes which means month.
Hope this helps! :3
