One part of writing English that people often get wrong is the use of the apostrophe ('). There are two main uses for an apostrophe.
1) Ownership/ Association
The most common use of the apostrophe is to show possession. Adding 's to the end of a word shows that the next word is owned by the first or associated with it. For example, the phrase Mark's textbook means that this is the textbook owned by Mark. If the word is a plural that normally ends in s, such as when more than one person owns something, the apostrophe is added after the s'.
The shop owners' meeting is at 9:00.
Thomas's homework was copied. (Next week will have more on this example.)
The girls' school needed painting.
When we are talking about grammar, possession does not have to mean ownership. In the above examples, Mark may own his textbook. The meeting is an event and it is not really owned by anyone. However Mark's school is associated with him, not owned by him. The apostrophe shows association, not strictly ownership.
Mary's calculator was lost.
Alex's cookbook is very old.
Tom's class is in room 2.
Jane's singing is improving.
The only words that don't1 need an apostrophe to show ownership or association are the words where it is part of the meaning: his, hers, its2, ours, yours, theirs meaning 'belonging to him, her, it, us, you, or them.'
1: See: Spelling left out below
2: See: its or it's below
2) Spelling left out (contraction)
In order to speak, and write faster, people often shorten words. An apostrophe is used to show where the missing letters are.
Tom 'n Suzie are dating. (and)
I'll do it soon. (I will)
I don't want to do homework. (do not)
Hong Kong returned to China in '97. (1997)
its and it's
When possession and contraction cause confusion...
Two words that can cause great confusion are its and it's. The first word refers to something that belongs to it, and would usually have an apostrophe. However if it did the spelling would be the same as it's. Having the same spelling and pronunciation would be even more confusing.
The dog was chasing its tail.
The statue was knocked off its pedestal.
An apostrophe is used when the word is short for it is. (or it has) This apostrophe shows that a letter or letters have been left out.
It's (it is) a lot of homework.
It's (it has) been a long time since we haven't (have not) had homework to do.
Apostrophe use gives the reader information about the subject and its misuse can create confusion.
Boy's die – The boy owns a die. (die: The singular of dice)
Boys die – More than one boy is dead.
Can't – Can not
Cant – A type of slang or wordy, hypocritical speech
Most of the time we can guess the meaning without an apostrophe, but they are used to make our meaning clearer. It prevents confusion.