Auxiliary verbs

These auxiliary verbs are used as follows:
(1) to form continuous tenses (be), and perfect tenses (have)
(2) to form the passive voice (be)
(3) to form questions
(4) to form negative statements
(5) to give emphasis
(6) to make short answers to questions and to avoid repetition
(7) to form question tags

Auxiliary verbs are often contracted when they are used in statements. This means they are not always fully pronounced. I am becomes I'm
She has becomes She's
They had becomes They'd
The dog has becomes The dog's

Compare the following: He's seen it already.
He'd seen it already. He'd worked all day.
He worked all day.
He's taken a course.
In this example, the past participle taken tells us that the auxiliary is has.
He's taking a course.
Here, taking tells us that the auxiliary is is.
However, the pronunciation of taken and taking is quite similar in fast speech.

Contractions are used in written storytelling, both in dialogue and narrative. He couldn't remember where he'd seen her before. Narrative
'There's been an accident,' she said. 'You'd better call the police.' Dialogue.
They are not written in the formal styles used for business reports, academic essays and so on.
It has been in use in the UK since 1988; however, it will not be introduced elsewhere as it is not compliant with the latest safety regulations. Report

Auxiliaries are not usually stressed and, as we have seen, are often contracted. However, in the following situations, it is normal to stress an auxiliary. In these cases, the auxiliary is never contracted. You haven't done your homework.
I have done my homework.
(Emphasis for contradiction)
Did you lock the door?
Yes, I did.
Short response Have you seen the film?
I haven't but Peter has.
Avoiding repetition