it’s – its
It’s with an apostrophe is short for it is. The short form is not used in formal writing:
Its without an apostrophe is a possessive pronoun:
The Academy will hold its next meeting in Boston.
who’s – whose
Who’s is short for who is and is not used in formal writing:
Who’s responsible for this?
Whose means of whom or of which:
Novak, whose recent paper on …
you’re – your
You’re is short for you are and is not used in formal writing:
Your is a possessive pronoun:
Your paper has been accepted for publication.
we’re – were – where
We’re is short for we are and is not used in formal writing:
Were is the plural of was:
We were very surprised at the results: they were completely unexpected.
Were is also used with you, whether singular or plural:
You were right, Professor Clever Clogs.
Professor Clever Clogs and Dr Smarty Pants, you were both wrong.
Where means at which place:
Where are you giving your paper?
they’re – their – there
They’re is short for they are and is not used in formal writing:
Their is a possessive pronoun:
Watson and Crick made their breakthrough in Cambridge.
There means that place:
We should go there one day.
There is also used in there is and there are:
There are several possible explanations for …