The apostrophe probaly causes more grief than any of the other punctuation marks put together!
The problem nearly always seems to stem from users not understanding that the apostrophe has two very different (and very important) uses in English.
to show possession and ownership - e.g. Jack's car. Mary's father.
to indicate a contraction - he's (he is), we're (we are), they're (they are)
These two examples show the apostrophe being used for possession (sentence 1) and contraction (sentence 2)
Colombia's coffee exports have risen steadily over the past decade.
Colombia's one of the main coffee producing countries in the world.
The POSSESSIVE APOSTROPHE
In most cases you simply need to add 's to a noun to show possession:
a ship's captain, a doctor's patient, a car's engine, Ibrahim's coat, Mirianna's book.
Plural nouns that do not end in s also follow this rule:
the children's room, the men's work, the women's club
Ordinary (or common) nouns that end in s, both singular and plural, show possession simply by adding an ' after the s but proper nouns (names of people, cities, countries etc.) can form the possessive either by adding the 's or simply adding the ':
a. The Hughes' home (or the Hughes's home), Mr Jones's shop (or Mr Jones' shop), Charles' book (or Charles's book)
b. the ladies' tennis club, the teachers' journal, the priests' church (note that the priest's church would only be refering to one priest while the priests' church refers to a group.)
General notes: Many people want to know how to form the possessive of their own name when it ends in an 's' or when refering to the whole family, e.g. The Jones' children.
Today it is no longer considered incorrect to use either form (Jones's or Jones') and many large organisations now drop the ' completely (e.g. Barclays Bank, Missing Persons Bureau) when publishing their name.
The APOSTROPHE for CONTRACTION
The most common use of contracted apostrophes is for:
has'nt = had not
can't = can not
there's = there is
mustn't = must not
I'm = I am
it's = it is
let's = let us
I've = I have (also they've, we've)
she's = she has or she is (also he's)
it's = it is (a contraction) while its = possession
who's = who is (a contraction) while whose = possession