Apostrophes

The apostrophe is used for two things only:

To signify two words joined: What is = What's

To signify possesion, here's the confussion:

Your bar would be "Harper's Bar" because there's only one "Harper".

If you were all nuns it would be "Nuns' Bar" - Many nuns - the apostrophe is used after the "s"

or

"At the zoo, the children were

Just to add to Jason13_32's reply, there is another use for apostrophe that MLA recommends. Although at present, an apostrophe is no longer needed for plural numbers, letters, abbreviations, and words mentioned as words, it makes sense to put one on some, if not on all, of them.

Take this for example:

A's is not as vague as As.

This is what I know:

If the family name is Harper, then the correct way to write it would be Harper's.
If the family name is Harpers, then Harpers's would be the correct one.

If the noun is singular and ends in s, then add 's.
If the noun is plural and ends in s, then add ' only.

I would like to clear the last of the two rules I posted just above this post.

...
If the noun is plural and ends in s, then add ' only.

what about James? James's or James'?

It's James's, Maj.

You are probably right; but what about Cervantes' Don Quixote or Socrates' ideas?

I think there are exceptions, and those two are some of the exceptions. According to Michael Swan in Practical English Usage, sometimes we add an apostrophe to a singular noun ending in 's' in older and foreign names. But just sometimes, Dickens is still written Dickens's novels. Tough question, Maj.

I am not entirely sure, but I guess it would be Cervantes's, Don Quixote's, or Socrates's ideas. You can also remove the 's from Cervantes's and Socrates's.

The rule is if the non is singular and ends in s, then you should add 's. But if it will sound awkward, then you may choose to omit it. Either one is acceptable.