Do not use the pronouns "they" or "their" when referring to a collective noun or an indefinite pronoun. Collective nouns include the terms "all," "everyone," and "everybody." They are collective because they refer to groups of people. Indefinite pronouns such as "each" and "someone," like collective nouns, do not indicate a specific gender . Do not substitute the pronoun "they" for a collective noun or an indefinite pronoun. Instead, try to avoid the need for the pronoun if possible. One way to avoid the pronoun problem is to substitute the word "the" or "a." Use "he or she" or "his or her" as an alternative only if absolutely necessary; this option almost always can be avoided by rewriting the sentence.
Explanation: When you are uncertain about using "I" or "me," you may want to ask yourself which word you would choose if writing in the third person. For example, it is easy to conclude that "the judge threw the gavel at her" is correct, while "at she" would be incorrect. Since the pronouns "me," "him," and "her" all are used as objects, you know to use "me" in the first person whenever "him" or "her" would be appropriate in the third person.
What's the difference between theirs and their's ?
Theirs is the third person plural possessive pronoun - it replaces "their" + noun.
Is this yours or theirs?
He found a book - is it theirs?
I can't find my keys, but theirs are on the table.
Theirs is a better idea.
Theirs is over here.
Though you may see their's written even by native speakers, it is incorrect. Theirs should never have an apostrophe.
The Bottom Line
The idea that theirs needs an apostrophe comes out of the fact that on virtually every other word, 's indicates possession, so English speakers sometimes think theirs should be spelled their's . However, this is always incorrect - theirs is the only correct spelling
Remember that the third person plural possessive adjective is their : They've sold their house . Don't confuse it with the adverb there , 'in that place' or with they're , which is the shortened form of 'they are'.
The third person plural possessive pronoun is theirs (not their's): If this is theirs, they'd better take it .