This is the easiest, and most annoying mistake to make, I think. And sometimes it could just be a slip of the finger that misses - or hits - the apostrophe button. But the thing is, it's really easy to fix this in your manuscript without pulling out all your hair. Okay, it'll take time. But it's worth it, right?


First, here's how the words work in their most common usage:

THEY'RE / THEIR / THEIRS / THERE / THERE'S


They're = contraction of "they are"
      They're inside the house.
Their = possessive pronoun - belongs to someone
      Their house is on the hill.
Theirs = predicate pronoun, possessive of "they"
       That house is theirs.
There = adverb
      The house is over there...or ... Hi there!
         = pronoun
      There is no one in that house.
There's = contraction of "there is" or "there was"
       There's another house next door. 

They're my friends. = correct
Their my friends. = incorrect
There my friends = incorrect in this usage
(Using "There, there, my friend." as an expression meant to calm someone is entirely different, and acceptable.)

That's their house. = correct
That's they're house. = incorrect
That's there house = incorrect

The house is over there. = correct
The house is over there. = incorrect

IT / IT'S / ITS

It = pronoun
       The teacher told the class it had to stay late.
It's = contraction of "it is"
       It's time to go home.
Its = possessive pronoun for "it"
      You can't judge a book by its cover.

It's one o'clock = correct
Its one o'clock = incorrect

Its colour is blue = correct
It's colour is blue = incorrect

Make sense? Okay. Now go to your book and do a Search/Replace. Yes, it'll take time. But the incorrect usage of those words is a definite flag for editors/agents.


There. You see? It's not their fault they're confusing. There's no easy way to learn it (English) or its meanings without practice. Just ask someone who speaks a different language. They'll say theirs is much easier!