Guide to English grammar

55 Weak forms and short forms

1 Strong and weak forms

A weak form is a spoken form such as the pronunciation of am as /m/ instead of /æm/. Weak forms are normal in speech. A short form is a written form, such as 'm instead of am in the sentence “I'm sorry”. We use short forms in informal writing.

Spoken Written

Strong /æm/ Full am

Weak /m/ Short 'm

In speech many words have both strong and weak forms. We use the strong form only in very careful speech, or when the word is stressed.
A practical guide to English grammar 56
2 Full forms and short forms
1 In informal writing, some words have a short form.
Fit a gas wall heater and you'll stop shivering. It'll warm up your bedroom so quickly you won't need a towel. It fits snugly and safely on the wall. And, because it's gas, it's easy to control and very economical.
(from an advertisement)

Full form Short form

It is easy to control. It's easy to control.

In the short form, we miss out part of a word and use an apostrophe instead. We do not leave a space before the apostrophe.
The short form corresponds to the spoken weak form: /itz/ instead of /it iz/. We use short forms in informal writing such as a letter to a friend. They can also be used in direct speech - in a film script or play, for example, when speech is written down. Full forms are used in more formal writing.

NOTE
We cannot use a short form when the word is stressed. (NOT Yes, it's) as a short answer. But we can use unstressed n't in a short answer, e.g. No, it isn't.

2 In short forms we use 'm (= am), 're (= are), 's (= is/has), 've (= have), 'd (= had/would) and n't (= not) in combination with other words. These are the main short forms.

Pronoun + auxiliary verb I'm you're we're they're he's she's it's I've you've we've they've I'd you'd he'd she'd we'd they'd I'll you'll he'll she'll it'll we'll they’ll Here/There/That+ auxiliary verb here's there's there'll there'd that's Question word + auxiliary verb who's, who'll, who'd, what's, what'll, where's, when's, how's Auxiliary verb + not aren't isn't wasn't weren't haven't hasn't hadn't don't doesn't didn't won't wouldn't shan't shouldn't couldn't mightn't mustn't needn't oughtn't daren't can't

A short form can also be with a noun, although this is less common than with a pronoun.
The bathroom's cold. This heater'll soon warm it up.

NOTE

1. 
   The short form 's can mean is or has. And the short form 'd can mean had or would.
   

It's a big house. (= It is ...)
It's got five bedrooms. (= It has ...)
If you'd asked, you'd have found out. (= If you had asked, you would have found out.)

2. 
   Sometimes we can shorten a form with not in two different ways. The meaning is the same. But “I am not” has only the one short form “I'm not”.
   

It is not... = It isn't... / It's not...
You will not ... = You won't .../ You'll not...

3. 
   In non-standard English there is a short form ain't (= am not/is not/are not/has not/have not).
   

That ain't right. (=That isn't right.)
A practical guide to English grammar 57