John Steinbeck'sOf Mice and Menhas, at its central core, very strong themes that certainly combine well with the tone, language and sentence structure that color the story. The story of two unfortunate men that are bound for life, the novel treats other themes ranging from the loss of the American Dream, loyalty vs. duty, discrimination, loneliness, insecurity, and actually, hope.
This being said, we can sense the tone of the novel even from the very beginning, when we learn that the setting of the story is the town of Soledad, which is the Spanish word for "loneliness". Already you can set the tone: doomed, sad, tragic, and helpless. We see this in most of the characters. In George's inevitable choices in life, in Lennie's final outcome, in Crooks's lifestyle, in Candy's issues, and even in Curley's wife's behavior. The tone does change with the character of Slim, who provides the lighter side to an otherwise dark read.
Being that the novel's character are rogue, male field hands, we will not expect the language to be complex or researched. George's liberal use of expletives and insults to call Lennie also denote the slang of the country and a very simple sentence structure filled with often mispronounced or incomplete words. Lennie's character uses language that would fit a man who is mentally challenged, and is constantly interrupted by his own double guessing. The men, in general, use strong language for their regular use. We specially see this in their description of Curley's wife as a "tart". Moreover, the men's angst is definitely is ever-present in their choice of words. They are often rough, insulting, matter of fact, simple, and straight to the point.
Finally, the sentence structure, as already discussed, is properly fit for language that needs to be spoken quickly, defensively, offensively, or in a reactionary way. The men in the story are not people who live peaceful and fulfilled lives. They offend each other, defend each other at other times, call each other out, protect, watch out for, or scold. This is why Steinbeck uses creative license to make sentences that do not follow the traditional researched ways: they are simple, clear, and quite transfer the messages that the men wish to send across quite straight.