Apostrophe

The most important use of the apostrophe in English is in contractions, constructions in which a letter or group of letters is elided.

Examples of Usage of Apostrophe:

Between a noun and following verb form

Between a pronoun and the following verb forms: am, are, is, have, has, had, will and would:
I’m, she’s (she is or she has), you’ve, they’d (they had or they would)

Between auxiliary verbs:

Between all auxiliary verbs except am and not:
aren’t, isn’t, wasn’t, weren’t, haven’t, hasn’t, don’t, doesn’t, didn’t, can’t, couldn’t, shan’t, shouldn’t, won’t, wouldn’t, mustn’t

Other cases for Auxilliary verbs:

Because the verbs is and are can be contracted with both the personal pronouns and the adverb not, negative constructions using these elements can be expressed in two ways:

for example,

You aren’t taking tea = You’re not taking tea;
She’s not here = She isn’t here.
In informal style, apostrophes are used to form contractions between the words

how, when, where, why, who, what and that and the auxiliaries is, has, have, did, will and would.

However, not all combinations are possible.

The words how, when, where, why, who, what and that can all contract with is:
How’s he doing?
When’s the meeting?
Where’s your brother?
Why’s that meeting conducted?
Who’s that man?
What’s going on?
That’s ridiculous.

The auxiliaries have and has are generally limited to contractions with how, where, who and what;
has can also be contracted with that:
How’ve you been?
Where’ve they gone?
Who’ve they seen?
What’ve we got here?
Where’s he gone?
Who’s fallen?
What’s he done?
That’s been used.

The auxiliary did can be contracted with how, where, why and who:
How’d (How did) you get to his home?
Where’d they take my clothes?
Why’d you lie to me?
Who’d you see?
The auxiliary will can be contracted with who, what and that:
Who’ll know?
What’ll happen?
That’ll be nice.
The auxiliary would can only be contracted with who:
Who’d like more cake?

Other examples for the use of apostrophe

Other common contractions include:
bo’s’n (boatswain), fo’c’s’le (forecastle), ha’penny (half-penny), jack-o’-lantern (jack of the lantern), ma’am (madam), o’clock (of the clock), rock ‘n’ roll (rock and roll), will-o’-the-wisp (will of the wisp) and young’un (young one)

In genitive cases:

Ali's pen, Ahmad's book

Precautions for using contractions/apostrophe:

Contractions are nearly always used in oral English and in written English they are generally considered to give a more relaxed and informal tone to the writing; they should therefore be avoided if this is not the intention of the writer.