If someone is reciting Kerias Shema (without the Brochos- His shul will daven past the zman) and in between the perakim he makes a hefsik which is not permitted. Does he have to (or can he) start from the beinning of Shma? Or he fulfilled his obligation for the part that he said already but he has to finish the rest of shma and it won’t be in the best way possible?
If somebody recites kerias shema before davening, the general rules of hefsekim apply, and one must be wary of making a forbidden interruption during recitation, or between paragraphs (Biur Halachah 66:5, s.v. ve’eilu; see Daas Torah 66:1, who cites a dispute of rishonim concerning this matter).
If one makes an intentional interruption during kerias shema, one should preferably start from the beginning of shema. If this is not possible, one should carry on from the place where one made the interruption (Mishna Berurah 65:1, and Biur Halachah).