Contractions

Common contractions like can't, don't, isn't, won't, shouldn't, and it's are fine to leave in the abbreviated form. Spelling them out in their full form presents a more formal tone and takes up space.

However, there are two occassions when contractions must not be used:

• In conjunction with a product or corporate name

INCORRECT: Microsoft® Word's® one of the programs most used by writers.
CORRECT: Microsoft® Word® is one of the programs most used by writers.

• Between a noun and verb

INCORRECT: The Writergrrls'll have a meeting next week.
CORRECT: The Writergrrls will have a meeting next week.

Common Errors with Contractions

There are two common errors made with contractions.

• it's (it is) vs. its (possessive form for it)
• they're (they are) vs. their (possessive form for they) vs. there(a place)

Taken from Microsoft Style for Technical Publications