Contraction represents a special form of elision, one that involves fixed grammatical patterns and which has become so established that even language purists don’t object to its use in spoken language. Contractions are also accepted in written representations of spoken materials and to some extent even in ‘proper written English’ nowadays.
One very common form of contraction involves different forms of the auxiliaries be, have, will & shall.
The other type main type of contraction involves a reduced form of the negation particle not in conjunction with an auxiliary as in: isn’t, doesn’t, don’t, hasn’t, haven’t, won’t, shan’t, can’t, wouldn’t, couldn’t, shouldn’t, oughtn’t, needn’t, mustn’t, daren’t.
Another contraction of the same kind, that is still stigmatised, is ain’t, which is extremely versatile in that it may represent all present tense variants of either be or have. A tentative reconstruction of how this came to be could look like this, assuming is and have as base forms:
At least the two forms [ɪnt] and [ant] still exist in the local Lancashire accent, which makes this reconstruction somewhat likely because we can assume similar forms for at least some other accents.
Contractions involving weak forms of pronouns are relatively rare, although it has been claimed that the Anglo-Saxon genitive s in e.g. John’s may actually historically have developed out of John his. One commonly accepted contraction of this type, however, is let’s, whereas forms like giz (representing both give us and give me) tend to be restricted to regional or lowerclass accents.
An odd one out in terms of contractions is dunno to represent (I) don’t know because it involves three words, rather than just two.
Knowles, G. (1987). Patterns of Spoken English: an Introduction to English Phonetics. London: Longman.
Roach, P. (2009). English Phonetics and Phonology: a Practical Course (4th ed.). Cambridge: CUP.