A contraction is a short form that uses an apostrophe (to indicate that letters have been removed) to combine either the subject and an auxiliary verb, or a auxiliary verb and the word not in its contracted form (-n't).
Contrary to what many people, especially those of older generations, claim, contractions are common and correct in informal speech and writing. They are, however, not usual in formal writing, such as a PhD thesis. In technical writing contractions now are fairly common. See examples: 
They are commonly used with pronouns (in affirmative forms) and with auxiliary verbs (in negative and interrogative forms). Likewise, they are commonly used with be (in the present tense), and after pronouns, "wh" words, and there/here (see subsections below):
- I'm sorry!
- They're waiting downstairs.
- Who's that?
- There's jelly for dessert!
- I am → I'm
- You have → you've
- There is → there's (see section below)
- It is → it's (see section below)
- I would → I'd
- She will → she'll
- I am not = I'm not (the form I aren't is not standard)
- You have not = You haven't
- They are not = They're not or They aren't
- It is not = It's not or It isn't
- I would not = I wouldn't
- She will not = She won't
- We cannot = We can't (see section below)
- Am I not? = Aren't I
- Do I not? = Don't I?
- Have I not? = Haven't I?
- Will I not? = Won't I?
Note: The above contractions are also common in negative questions, such as Haven't they replied yet?
In several cases there is a change to the vowel sound - this may vary with dialect.
- we are → we're /wɪər/
- they are → they're /ðeər/
- you are → you're /jʊər, jɔːr/
- cannot → can't /kɑːnt,BrE kænt/AmE
- shall not → shan't /ʃɑːnt/BrE
- do not → don't /dəʊnt/
- will not → won't /wəʊnt/ (an Old English contraction of wonnot)
In a few cases a /z/ sound is changed to /s/.
- it is → it's /ɪts/
- it has → it's /ɪts/
- that is → that's /ðæts/
- what is → what's
In some cases an unwritten sound is pronounced.
- does not → doesn't /ˈdʌzənt/
- has not → hasn't /ˈhæzənt/
- is not → isn't /ˈɪzənt/
- must not → mustn't /ˈmʌsənt/; Note that the "t" is not pronounced in the contraction
- could have → could've /ˈkʊdəv/
- should have → should've /ˈʃʊdəv/
- would have → would've /ˈwʊdəv/
- must have → must've /ˈmʌstəv/
- it had → it'd /itəd/
- it would → it'd /itəd/
A silent e may or may not become pronounced.
- have → haven't /ˈhævənt/
- are → aren't /ɑːrnt/
The verb be may sometimes be contracted with the word preceding it or the word following it:
|(I am)||I'm _____________, aren't I?||I'm not||* see note 1 below|
|(We are)||We're _________, aren't we?||We're not||We aren't|
|(You are)||You're _________, aren't you?||You're not||You aren't|
|(She is)||She's __________, isn't she?||She's not||She isn't|
|(He is)||He's ___________, isn't he?||He's not||He isn't|
|(It is)||It's ____________, isn't it?||It's not||It isn't|
|(They are)||They're _______, aren't they?||They're not||They aren't|
- Note 1
- * is not correct. I'm not is the standard contraction. Likewise, for questions, aren't I? is the standard contraction, although amn't I? may be used in some dialects.
- Note 2
- The contraction ain't, for am not, are not and is not, although widespread in the 18th century as a contraction for am not and still normal in many dialects and informal speech, is however not standard English and should not be used in formal or written contexts.
Use of 's
Confusingly for many students, at least until they get the hang of it, 's is used as the contracted form of is and has:
- It's getting late. (is)
- It's been a lovely evening. (has)
- He's finished. (has)
- He's finished. (is)
- She’s ill. (is)
- She’s gone. (has)
Use of 'd
Normally 'd replaces either would or had. The confusion is possibly less frequent than the case of 's (see above).
- I'd been waiting for a long time. (had)
- I'd like to see them again. (would)
- I’d see a doctor if I were you. (would)
- I’d never seen her before. (had)
Sometimes 'd stands for did as in "Where'd they go?", but this is not considered standard English.
It is very important that students take care with the vowel change in "can't" as the final "t" is often lost in phrases such as "I can't tell the difference" and the only way for a listener to decide if the speaker has said "can" /kæn/ or can't" /kɑːnt/ may be through the vowel change (note that in General American there is no vowel change). To further complicate the issue, the verb "can" in "I can tell the difference" would probably be pronounced in a different weak form anyway, as /kən/, thus giving students three forms to contend with (/kæn, kən/ and /kɑːnt/BrE)
I can tell - I can't tell
- British English:
- strong "can": /ˌaɪ kæn ˈtel, ˌaɪ kɑːn(t) ˈtel/
- weak "can": /ˌaɪ kən ˈtel, ˌaɪ kɑːn(t) ˈtel/
- American English:
- strong "can": /ˌaɪ kæn ˈtel, ˌaɪ kæn(t) ˈtel/
- weak "can": /ˌaɪ kən ˈtel, ˌaɪ kæn(t) ˈtel/
In any case, it's always useful to remind students that they can always fall back on the "full" form cannot in order to avoid any possible confusion.
We often use contractions (especially ’s for is or has) after question words (who, what, etc) and after that, there, here:
- Who’s coming with me? (is)
- Who’s finished? (has)
- Here’s my bus. (is)
- That’s my book. (is)
- What’s happened? (has)
See main article Confusable
In some cases two spoken forms are pronounced identically but are written differently (homophones).
Students may mistake the contraction ’s for a possessive ’s (or vice versa):
- Peter’s here (he is here)
- Peter’s car (the car belongs to Peter)
Some words and contractions are often confused, in particular, possessive pronouns do not use an apostrophe:
Your ≠ You're
- You're going in your car? (contraction of You are going in your car?)
- Is often incorrectly written:
- * or *
Whose ≠ Who's
- Who's going in whose car? (contraction of Who is going in whose car?)
- Is occasionally incorrectly written:
Its ≠ It's
- Where is the dog? It's gone to its kennel. (contraction of ...It has gone to its kennel.)
- Is often incorrectly written:
Could've, not *
If in doubt use the expression without contraction.
Other homophones involving contractions include the following: aisle - I'll - isle; eyed - I'd; heel - he'll; their - there - they're; we'd - weed; we'll - wheel.
Some words are always written with an apostrophe: o'clock, jack-o'-lantern, will-o'-the-wisp and Hallowe'en (can also be written Halloween).
- Mental Floss, Why Does "Will Not" Become "Won't"?
- "aren't" Oxford Dictionaries
- "ain't" Oxford Dictionaries
- Random House Dictionary, 'd.