Like many of Mark Twain's novels, Huckleberry Finn is written in a style that encompasses the vernacular of the time, with focus on the dialects of each character and their respective backgrounds. Twain mentions these dialects in his preface, explaining that he had personal knowledge of each dialect, and this informs the linguistic devices of the narration. The book is told in first-person, with both past- and present-tense used in appropriate spots. In the first two paragraphs, the voice and style is laid out:
You don't know about me without you have read a book by the name of The Adventures of Tom Sawyer; but that ain't no matter. That book was made by Mr. Mark Twain, and he told the truth, mainly. There was things which he stretched, but mainly he told the truth.
The Widow Douglas she took me for her son, and allowed she would sivilize me; but it was rough living in the house all the time, considering how dismal regular and decent the widow was in all her ways; and so when I couldn't stand it no longer I lit out. I got into my old rags and my sugar-hogshead again, and was free and satisfied.
(Twain, The Adventures of Huckleberry Finn, eNotes eText)
Huck finn is a simple boy, and so his narration is equally simple. While other characters, such as the Duke and Dauphin, speak eloquently to hide their ignorance, Huck usually describes things as they are, with little embellishment. Linguistic devices present in the first two paragraphs include jargon, which covers the many odd phrases and grammar used by Huck and others. The double-negative "ain't no" or "couldn't... no longer" was typical of the time, and so appropriate. In the first paragraph, the famous lines "...told the truth, mainly. There was things..." is written in an almost poetic way, and is read and spoken easily and with rhythm; this is an example of poetic expression inside prose.
In the second paragraph, Huck's living arrangements are presented as an oxymoron; while "sivilized" people would think that Huck's new life is normal, he sees it as stifling and boring, referring to "regular and decent" ways as "rough." This contrast helps establish Huck's character as a free spirit. Similarly, he is "free and satisfied" wearing "rags" and living in a "sugar-hogshead" (a large barrel) which would be considered terrible poverty for many people.
One other example is the personification of Mark Twain as a real person in the book. Huck mentions that Twain "stretched" some things, showing him as a writer committed to good storytelling rather than pure fact. In this way, the story that follows can be assumed to be slightly "stretched" and not purely factual. Twain thus sets himself apart from Huck, whose narrative voice is unique. It is interesting to note that Tom Sawyer was not written in first-person; that book is more of a straight novel, with Twain as narrator commenting on events.