64:1. If one read the Shema out of order, (1) he has not fulfilled his obligation. This is in a case where he read (2) the verses out of order. However, if one read the paragraphs out of order, he has fulfilled his obligation, although one is not permitted to do this initially. [The three paragraphs are: a)"Shema Yisrael...v'Ahavta..."; b)"V'Hayah Im Shamoah..."; and c)"Va'Yomer...". - LC] This is because the paragraphs of the Shema (3) are not immediately adjacent to one another in the Torah [so there is no special reason to read them in order].
64:2. If one read a paragraph of the Shema (4) and made a mistake, if he knows where he erred (e.g., if one read the entire paragraph, but skipped (5) one verse in the middle) then he must return to the beginning of the verse in which the mistake occurred (6) and complete the paragraph from that point. If he does not know where he erred, then he must go back (7) to the beginning of the paragraph.
64:4. If one (9) has just read "Uchtavtam" [*] and does not know if he is at the "Uchtavtam" of the first paragraph or the second, then he should go back to the "Uchtavtam" of the first paragraph. This is only if he did not begin the verse "Lema'an yirbu yemeichem," [**] but (10) if did then (11) there is no need to go back, because this proves that he was finishing the second paragraph. [* - The first word of the verse "Uchtavtam al mezuzot beitecha uvisharecha" - And you shall write them on the door posts of your house and your gates. This verse is found in both the first and the second paragraph.] [** - So that your days will be many; the beginning of the verse following "Uchtavtam" in the second paragraph.]