MB 53: Properly - But this is not to say that he did not fulfill the obligation to recite the Shema at all, for we rule that the blessings [if not recited properly] do not invalidate the recital of the Shema. Rather, this means that his recital of the Shema is also not as it should be, and he should at least go back to say this blessing after praying the Amidah if he did not say it beforehand, as written above in 60:2. And in this case, it is a good thing to repeat the Shema as well (Birkei Yosef).
The Tur wrote in the name of the Talmud Yerushalmi that one must mention during "emes v'yatziv" [the blessing after the morning Shema] the Exodus from Egypt, the Kingship of G-d, the Splitting of the Sea, and the Plague of the First-Born, see there. And it is implied in the Rashba to Tractate Brachos, p. 11 that this invalidates [the blessing, meaning that one must repeat it if he failed to mention these things] even after the fact.
[The following paragraph may be very difficult for those who have never studied Talmud to understand. It is also of little practical relevance, because we all use printed Siddurim that provide texts that fulfill the above requirements. Even the Mishnah Brurah put it in parentheses... --YM]
(And this requires study, for it is written in Brachos 14b: [The excerpt begins in the middle of a discussion about the evening prayer] "[Objection:] But he must mention the Exodus from Egypt [in the evening]! [Answer:] He said thus [in the evening prayer, instead of "emes ve'emunah" (the first blessing after the evening Shema)]: 'We thank you, L-rd our G-d, that you took us out from the land of Egypt, and redeemed us from the house of slaves, and did for us great deeds at the sea, and we sang to you", i.e., he continues "Who is like you", etc. until the end of the blessing, as Rashi explained the text. According to this, he does not mention the Plague of the First-Born! And it is far-fetched to say that there is a difference between the morning and evening prayers. Perhaps they had a different text at the end of the blessing which mentioned the plague of the first-born. Also, from the Talmud Yerushalmi Brachos Chap. 1, Halacha 6, it seems to be implied that the requirement is only beforehand, but after the fact [if one said the blessing without mentioning these things] it does not invalidate the blessing. As for practical application, it requires study.)
If one substituted the evening blessing for the morning blessing or vice-versa, then if he realizes his error before mentioning G-d's name at the end of the blessing, he should go back to the beginning ["v'yatziv" or "ve'emunah"]. If he has already said G-d's name, he should complete the blessing and he need not go back. [Hayye Adam in the name of R. Manoach]
MB 54: And does not have time - This requires study, for who permitted him to make an interruption in the middle because he does not have time? Rather, one who does not have time should recite the Shema by itself without the blessings, and afterwards when he has time, he should recite the Shema, its blessings, and the Amidah together, as above in 60:2 (Magen Avraham)
In the book Nahar Shalom, he wrote that nevertheless the ruling of the Ramah is true and correct if one recited the Shema under the impression that he would be able to complete the entire prayer, and then after he recited the Shema he was forced by circumstances to stop. In this case, the Rama explains above that it is preferable that he stop after "Hashem Elokeychem emes", and afterwards he should begin from "v'yatziv" etc. when he is able to pray. However, if one knows beforehand that he will not have time to recite Shema and to pray, and he fears that the time of the Shema will expire before he will have time, the Rama will certainly agree with the Magen Avraham that he should recite the Shema by itself. And the Pri Mgadim explained similarly, see there.