An ELT Glossary : Weak Forms

Definition : The pronunciation of a grammatical word in English when in unstressed position.

Words affected in this way are grammatical items such as prepositionsconjunctionsauxiliary verbs, pronouns, quantifiers and articles. The strong form (or citation form) is how the word would be pronounced if it was said in isolation or in a stressed position in a sentence - for example, in the following exchange at is stressed and would be pronounced /æt/ : What are you looking at?

In an unstressed position in the sentence, however, and in normally rapid connected speech, at would have a different pronunciation - /ət/  :  I looked at what you'd done and...

Here are some more examples : 

Here are some more examples  

are: citation form =/ɑː/;  weak form = /ə/ 
somecitation form = /sʌm/; weak form=/səm/

What have you been doing?  
havecitation form = /hæv/; weak form= /əv / 
youcitation form =/ju:/; weak form= /jə/ 
beencitation form =/bi:n/; weak form= /bɪn/

When can we see them?  
cancitation form =/kæn/; weak form = /kən/  
we: citation form =/wi:/; weak form = /wɪ/
themcitation form =/ðem/; weak form /ðəm/

As you can see, weak forms involve a number of features common in connected speech - for example elision (the omission of a sound) as in have, where /h/ is likely to be elided. However, perhaps the most obvious change is the difference in vowel quality from the strong form - long vowels are replaced by short vowels - often the schwa /ə/  as in some or can, but also other short vowels: been /bi:n/ changes to /bɪn/ for example.

Why does this happen? English is a stressed timed language. The rhythm, and length of time that it takes to say an utterance depends on the number of stressed syllables it contains (and not the total number of syllables, as in syllable timed languages). The stressed syllables are said at regular intervals, and the unstressed syllables get "squashed in" between them - regardless of how many of them there are. 

This can create problems for learners, especially those from syllable timed languages who are expecting the unstressed words to be pronounced as clearly as the other words in the utterance. They may fail to recognise the weak form pronunciation or even to hear them at all.