The Laws of Krias Shema part 2 vol. 3, no. 22
In the order of the morning prayers, Krias Shema is preceded by two blessings and followed by two blessings, as is printed in all siddurim. If one recites Shema without the blessings he fulfills his mitzvah of reciting Shema but he is required to recite the blessings with Shema later on.
It is common for many congregations to recite Shema later than three hours into the day. In such circumstances, one should recite Shema before going to shul to fulfill his mitzvah and later on recite the blessings in shul with the congregation.
The blessings should be recited in the order printed in the siddur. However if one erred and recited them out of order he does not need to recite them again.
If one is in doubt whether or not he recited Shema he needs to recite it with its blessings. However if he is sure he recited the blessings and is only in doubt as to his recitation of Shema he recites only Shema.
If one accidentally recited the first blessing that precedes the evening Shema instead of the first blessing that precedes the morning Shema, he has to recite the proper morning blessing. If one recited the entire blessing properly but concluded the blessing with the conclusion for the evening Shema (hama’ariv aravim) he must repeat the entire blessing, unless he realizes his error and can say the correct ending (yotzer ham’oros) within a couple of seconds of his error.
The second blessing preceding Shema in the morning begins ahava rabah (according to the Ashkenazic custom) or ahavas olam (according to the Sephardic custom). If one accidentally recited the second blessing which precedes the evening Shema in the morning he does not need to repeat the proper blessing. (Note: if one is not sure if he recited the blessings for Torah learning or, for whatever reason, did not fulfill his obligation to recite these blessings, he may have in mind to fulfill his obligation with the recitation of ahava rabah.)
