Question

When "theirs" is used, should I use an apostrophe?

Answer 1

no

Answer 2

you only use apostrophe s when you say "their is" which does not exist

Answer 3

Possessive pronouns and adjectives never take apostrophes.

Possessive pronouns are used alone, without an accompanying noun: that one is mine, that one is yours, that one is his, and so on.

Possessive adjectives are used when the form precedes a noun: that is my car, that is your car, that is his car, and so on.

The possessive pronouns are --

mine, yours, his, hers, ours, theirs

The possessive adjectives are --

my, your, his, her, its, our, their

The reason this can be confusing is that you DO use an apostrophe with nouns in the possessive. In fact, it is the apostrophe which identifies the possessive form of the noun. The student's paper, the paper's thesis, language's idiosyncracies.

But the apostrophe used to show possession is entirely a noun thing. It doesn't come into play with other parts of speech.

The apostrophe can also be used to show contraction (can't = cannot, for example), and this adds another little twist to the story. Two forms that are consistency confused are "it's" and "its." The first is a contraction; the second is a possessive pronoun.

To distinguish them, remember that "it's" always always indicates a contraction --

it's = it is
or
it's = it has


*Examples*

I see that it's going to rain. It's really cloudy out. (it's = it is)

It's time to take a break. (it's = it is)

It's been good seeing you again. (it's = it has)

vs.

The cat is licking its paw. (Or: the cat is licking her paw, the cat is licking his paw.)

I love that song -- I know its lyrics by heart.

With its natural grace, the cat can be a delight to watch.