subject + auxiliary + main verb
General use of auxiliaries
The auxiliaries be, do and have have the following functions in English:
(1) To form certain tenses - continuous tenses (be); 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
Pronunciation: contraction of subject and auxiliary
Auxiliary verbs are often 'contracted' when the 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
This is common in fast speech and can cause problems for students because:
(1) Students find it difficult to hear which auxiliary is being used.
Compare the following:
'He's seen it already' and 'He'd seen it already'
(2) In the past perfect tense, the contracted auxiliary 'had' can be very difficult to hear:
'He'd worked all day'
'He worked all day'
(3) The contracted form of 'has' is the same as the contracted form of 'be' in the third person.
'He's taken a course'
It should be clear that the 's must be 'has' becuase it is followed by the past participle 'taken'. However, compare:
He's taking a course'.
The pronunciation of 'taken' and 'taking' is quite similar in fast speech.
Contraction of subject and auxiliary in written English
The contraction of subject and auxiliary is often written in novels and other texts where natural speech is recorded. However, these contractions are not written in the more formal written style used for business correspondence, academic essays and so on.
'We'd better get out of here'
(2) Formal letter
'I have sent the samples by courier'
When auxiliaries are stressed
In statements, as noted above, auxiliaries are often contracted and are not usually stressed.
However, in the following situations, it is normal to stress an auxiliary. In these cases, the auxiliary is never contracted.
(1) When using an auxiliary for emphasis:
'I have done my homework'
(2) When an auxiliary is used as a short response or to avoid repetition:
'I did' (In response to, 'Did you lock the door?')
'I haven't but Peter has' (In response to, 'Have you seen the movie?')