Weak Forms
When we talk about weak forms in the phonetics of English this regards a series of words which have one pronunciation (strong) when isolated, and another (weak) when not stressed within a phrase, e.g.
| a car | /ˈeɪ ˈkɑ:/ |
|
| I bought a car | /aɪ ˈbɔ:t ə ˈkɑ:/ |
|
Weak forms are usually distinguished by a change in vowel quality from a border position on the vowel quadrilateral to a central position. The vowel in a weak form is usually the schwa (ə). Weak forms are pronounced more quickly and at lower volume in comparison to the stressed syllables. They are also not central to changes in intonation.

Fig. 1. The change of position of vowel production for the articulation of weak forms.
There is a logical explanation behind the occurrence of weak forms: they are present in words which are necessary to construct a phrase yet, at the same time, do not communicate a large quantity of information, in other words, they are not content words. For example in the following phrase:
the most important words, those that are
central to the message, can be emphasised:
I went
to the hotel and booked a room
for two nights for my father and
his best friend.
/aɪ ˈwent tə ðə həʊ ˈtel ən ˈbʊkt ə ˈru:m fə ˈtu: ˈnaɪts fə maɪ ˈfɑ:ðər ən hɪz ˈbest ˈfrend/
You will notice that most of the unstressed words are pronounced with the sound /ə/: prepositions such as to and for, articles a, an and the, and the conjunction and. Auxiliary verbs frequently have weak forms.