A pronoun takes the place of a noun. A possessive pronoun takes the place of a noun while showing ownership.
That is my aunt and uncle’s house.
That is their house.
The house is theirs.
In the second two sentences, the pronoun replaces the name of the people we are talking about – my aunt and uncle.
Use the words their and theirs to replace the names of people who are being talked about. The pronouns their and theirs are possessive pronouns and show ownership.
It’s your turn. Which of these pronouns – their or theirs – would you use in this sentence? My sister and my baby brother share a bedroom. The bedroom is _____. Did you figure it out? Good for you!