Some are correct, others aren't. Our present is a pen. Theirs is balls. - No Our present is pens. Theirs is a ball. - Yes (suggesting multiple pens make up one present - like you received a box of pens) Our present is pens. Theirs are a ball. - No Our present is pens. Theirs are balls. - Yes Our presents are a pen. Theirs are balls. - No Our presents are pens. Theirs are a ball. - No Our presents are pens. Theirs is a ball. - Yes (you received multiple presents, they received one) Our presents are pens. Theirs is balls. - No Our presents are a pen. Hers is balls. - No Our presents are pens. Hers is a ball. - Yes