The apostrophe must be the most misunderstood and misused piece of punctuation in the English language. This is made worse by the fact that most people simply fail to understand what it does, and make it unnecessarily complicated. The result is that many people, in an effort to appear correct, use a scattergun approach, dropping in apostrophes every time the letter "s" ends a word, for plurals, possessives and contractions alike.
It's easy when you know one rule
Using apostrophes correctly is easy - once you know the rule! Yes, that's right. THE rule.
Despite what you may have heard, there is just one place where an apostrophe is used. Just one. And it's really easy to remember.
Use an apostrophe when letters are missing.
But "How can this be true?" I hear you cry.
How can there be just one rule which covers all uses of apostrophes? Well it's true. Use an apostrophe when letters are missing. It is as simple as that.
When we deliberately shorten a word or phrase we use an apostrophe to show that letters are missing. For example, "do not" becomes don't, "cannot" becomes can't and "you are" becomes you're. Simple enough, right?
Misuse often occurs where plurals are involved. Plural simply means more than one. So we see a sign that says "Parent's are asked to supervise their children". No letters are missing in this sentence; it is a request to more than one parent to look after their kids. The correct form is "Parents are asked to supervise their children".
But the children belong to the parents, you say. True no doubt, but the two words are not together in the sentence and the message is directed at parents, not children.
Parents children would need an apostrophe, but before or after the "s"?
To understand this we need to go back to old English. In the times of Chaucer, words that show belonging would end in "es" as they do today in modern German. For example, the English "The man's coat" is "Der Mantel des Mannes" (The coat of the man) in German. Note the -es ending on Mann to show possession.
The old -es possessive form in English is now missing but we use an apostrophe when letters are missing.
So to refer to one parent and his or her children, the old English would have been "parentes children". In modern English it is "parent's children".
To refer to all parents and all their kids the old English would have been "parentses children". The correct modern form being "parents' children".
Its and It's
People are often confused about this but the same single rule applies - Use an apostrophe when letters are missing.
If you are contracting "it is" then it becomes "it's" because the "i" is missing. For every other situation, there's no apostrophe. The other "its" is a genitive form of a possessive pronoun like his or hers. It does not have an apostrophe.