this is your book
this book is yours
Juz jasne? Jak nie ma rzeczownika, piszesz 'yours' (=twoja, wasza)
Inne przyklady
This is my book
this book is mine
this is her car
this car is hers
this is his house
this house is his (tu bez zmian, bo 's' juz jest na koncu slowa)
podobnie
ours, yours (w liczbie mnogiej), theirs