1. One who is reciting the Shema and made a mistake in the middle of a paragraph (e.g. he omitted a word or a verse), he must return to the beginning of that verse, and continues to the end of that paragraph. If he does not know exactly where he erred, he should return to the beginning of the paragraph about which he is unsure. (Shulchan Aruch and Mishne Berurah 64)
2. One who errs in the middle of the first paragraph, and does not remember where he erred, if it is clear to him that he recited the verses “Shema” and “Baruch shem” with the correct concentration, he should return to “Ve’ahavta”, but if he is unsure if he concentrated in those verses, he returns to the beginning. (ibid)
