How to use the apostrophe correctly

As a proofreader and translator, one of my main tasks is to correct English-language documents written by Germans. What surprises me is that these people tend to be more familiar with the mechanics of grammar than so-called "native speakers" are.

Indeed, I regularly come across some shocking examples of how poor the general level of literacy is in both the UK and the USA. For that reason, I have decided to devote an article to one particular irritant of mine - misuse of the apostrophe!

The apostrophe. Please use it correctly.

The first use of the apostrophe is to indicate possession. For example,

Emily's book (i.e. the book belonging to Emily)

Mr Smith's house (the house belonging to Mr Smith)

The company's turnover (the turnover of the company)

If you have a singular possessive noun that already ends with 's', you can either add an apostrophe on its own or an apostrophe + an extra 's' after it. For instance,

Charles' book or Charles's book 

are both acceptable forms. 

BUT if the possessive noun is a plural (i.e. more than one person or thing) that ends with 's', then just place an apostrophe after it, e.g.,

The girls' mother (the mother of the girls)

The McCanns' lawyer (the lawyer for the McCanns)

The students' room (the room for the students)

Two weeks' notice (a notice period of two weeks)

If the possessive noun has an irregular plural form which does not end with 's' (for instance, child → children, man → men), then add an apostrophe + 's', as you would if it was a singular possessive noun. e.g.,

The children's mother
The men's room

DO NOT use the apostrophe to form ordinary plurals.

One cat → two cats (not *cat's)

One teacher → two teachers (not *teacher's)

One holiday → two holidays (not *holiday's)

It's "Holidays". Yes, Madonna, I'm talking to you!

Please also note that you do not use an apostrophe if the possessive noun is "it".

The crocodile is known for its aggressive nature

The snowman has lost its head

The only time you use "it's" is when you wish to merge the words "it is" or "it has", e.g.

It's very cold outside (it is very cold outside)

It's got a long nose (it has got a long nose)

This brings me to the second use of the apostrophe - for contractions (i.e. shortened forms of words). The apostrophe is used to indicate the missing letter(s). Some frequently-used examples are below:

Can't = cannot

Doesn't = does not

Don't = do not

Shouldn't = should not
You're = you are

Cathy's ill = Cathy is ill

They're nice people = They are nice people

He's a very selfish person = He is a very selfish person.

On a final note, be careful not to confuse words like "he's"/ "his", or "you're/your" or "they're"/"there"/"their".

Remember:

He's = he is or he has, e.g., "he's a bricklayer"/ "he's got a new car".

His = belonging to him, e.g., "his house".

You're = you are, e.g., "you're very kind"
Your = belonging to you, e.g., "your money"

They're = they are, e.g., "they're very rich".

Their = belonging to them, e.g., "their car".

There = opposite of "here", e.g., "she lives over there"