oxford_guide_to_english_grammar

7 SPOKEN ENGLISH AND WRITTEN ENGLISH
55 Weak forms and short forms
A weak form is a spoken form such as the pronunciation of am as /m/ instead of
/æm/. Weak forms are normal in speech. A short form is a written form, such as 'm
instead of am in the sentence I'm sorry. We use short forms in informal writing.
Strong Weak
Spoken /æm/ /m/
Full Short
Written am 'm
1 Strong and weak forms
a
In speech many words have both strong and weak forms. We use the strong form
only in very careful speech, or when the word is stressed.