The house is now theirs, it was ours at one time but now it has been sold it and it is hers

And so we have such uses as “my house”, “our holiday”, “their problem”, etc., and “the house is mine”, “that holiday was ours”, “that problem is theirs”, etc.

Examples:

This is their house.

This is our dog.

This is her bag.

(These always have nouns/ noun phrases behind them.)

theirs, ours, hers..

Example:

This house is theirs.

This dog is ours.

This bag is hers.

(These have no nouns/ noun phrases following them.)

Hope this explanation is simple enough.

They are the objective possessive pronouns.

mine

yours

hers

his

its

ours

theirs

.

No explanation.