Siman 64
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א קְרָאָהּ לְמַפְרֵעַ, לֹא יָצָא, בַּמֶּה דְּבָרִים אֲמוּרִים, בְּסֵדֶר הַפְּסוּקִים, אֲבָל אִם הִקְדִּים פָּרָשָׁה לַחֲבֶרְתָּהּ, אַף עַל פִּי שֶׁאֵינוֹ רַשַּׁאי, יָצָא, לְפִי שֶׁאֵינָהּ סְמוּכָה לָהּ בַּתּוֹרָה.
1 Laws of One who Errs During the Recitation of the Sh'ma, containing 4 s'ifim One who read it out of order has not fulfilled [their obligation]. This applies with regard to the order of the verses, but if one advanced a section ahead of its peer, even though they are not allowed to do so [intentionally], they have fulfilled [their obligation] since it is not adjacent in the Torah [itself].
ב קָרָא פָּרָשָׁה וְטָעָה בָּהּ, אִם יוֹדֵעַ הֵיכָן טָעָה, כְּגוֹן שֶׁקָּרָא כֻּלָּהּ אֶלָּא שֶׁדִּלֵּג פָּסוּק א' בָּאֶמְצַע, חוֹזֵר לְרֹאשׁ אוֹתוֹ הַפָּסוּק וְגוֹמֵר הַפָּרָשָׁה, וְאִם אֵינוֹ יוֹדֵעַ הֵיכָן טָעָה, חוֹזֵר לְרֹאשׁ הַפָּרָשָׁה.
2 One who read a section and erred in it: if he knows where he erred, for example, he read the whole section but skipped one verse in the middle, he returns to the beginning of that verse and finishes the section. But if one does not know where one erred, he returns to the beginning of the section.
ג טָעָה בֵּין פָּרָשָׁה לְפָרָשָׁה, שֶׁהוּא יוֹדֵעַ שֶׁסִיֵּם פָּרָשָׁה וְאֵינוֹ יוֹדֵעַ אִם רִאשׁוֹנָה אִם שְׁנִיָּה, חוֹזֵר לְפָרָשָׁה רִאשׁוֹנָה, וְיַתְחִיל וְהָיָה אִם שָׁמוֹעַ.
3 One who erred in-between sections, in that he knows that he completed a section, but does not know if it was the first ("v'ahavta") or the second [section] ("v'haya im shamoah"), he returns to [the end of] the first section and then begins "If you will listen...".
ד הָיָה עוֹמֵד בְּוּכְתַבְתָּם, וְאֵינוֹ יוֹדֵעַ אִם בְּוּכְתַבְתָּם שֶׁבְּפָרָשָׁה רִאשׁוֹנָה אִם בְּוּכְתַבְתָּם שֶׁבְּפָרָשָׁה שְׁנִיָּה, חוֹזֵר לְוּכְתַבְתָּם שֶׁבָּרִאשׁוֹנָה, וְהָנֵי מִילֵי שֶׁלֹּא הִתְחִיל לְמַעַן יִרְבּוּ יְמֵיכֶם, אֲבָל אִם הִתְחִיל לְמַעַן יִרְבּוּ יְמֵיכֶם, אֵין צָרִיךְ לַחֲזֹר דְּסִרְכֵיהּ נָקַט וַאֲתָא:
4 If one is at the point of "and you shall write them" [which appears in both of the first two sections] and he does not know if it's the "and you shall write them" that is in the first section or the "and you shall write them" that is in the second section, he returns to the "and you shall write them" that is in the first [section]. And this is where one did not begin "so that your days shall be multiplied" [which immediately follows only in the second section], but if one did begin "so that your days shall be multiplied", there is no need to go back in that one can assume one followed one's routine and continued.