In Of Mice and Men, Steinbeck uses a non-standard form of English throughout the book. The dialect is that of the itinerant worker. This use of dialect and colloquial language helps to add to the realism of the story and to create a strong impact upon the readers. It is also a way to show that the characters have received little education since people who use non standard English may often be considered / regarded as poorly educated.
Here are some examples of nonstandard English that can be seen throughout the novel: