Weak forms occur in stress-timed languages such as English when the word itself is not stressed. This makes such words tricky for untrained listeners to identify as they may well be expecting the stressed strong form. Normally the weak form is identical to the strong form, with the vowel changed by schwa /ə/. For example the strong form of "could" is /kʊd/ and its weak form is /kəd/.
Native speakers sometimes think they are using the language badly when they use weak forms but this is not the case. They are a perfectly natural part of the English language, and the schwa is the most common sound in the language.
Teachers, especially, should continue to use weak forms. If teachers only use strong forms when they speak to students they will be giving them a completely erroneous impression of the way the spoken language sounds, almost as bad as pronouncing the "t" in "listen". This is a very bad form of teacher talk which teachers need to be very much aware of, as the result of teachers constantly using strong forms will be students who are able to understand the teacher but nobody else.
The main words with weak forms are:
a /eɪ, ə/ - am /æm, əm, m/ - an /æn, ən/ - and /ænd, ənd, ən, n/ - are /ɑr, ər/ - as /æz, əz/ - at /æt, ət, ɪtAmE/
be /biː, bɪ/, but /bʌt, bət/
can /kæn, kən/, could /kʊd, kəd/
Informal shortened forms of "because":
- UK: cos, ‘cause /kɒz, kəz/
- US: ‘cause, cos /kɔːz, kʌz, kəz/
do /duː, dʊ, də/ - does /dʌz, dəz/
had, has, have, he, her, him, his
of, or,AmE our
shall, she, should, some
than, that, them, there
Strong form: /ˈðiː/
- before a consonant: /ðə/
- before a vowel: /ðiː, ðɪ/
Strong form: /tuː/
- before a consonant: /tə/
- before a vowel /tuː, tʊ/
Negatives don't have weak forms
Negative contractions don't have weak forms. For example, "could" has the weak form /kəd/ but "couldn't" only has the form /ˈkʊdənt/. Similary "are" has the weak form /ər/ but "aren't" only has the form /ɑːrnt/ (identical to "aunt" in Received Pronunciation).