MB 32: After its prescribed time - That is, after three variable hours [see explanation after this MB], the time for the commandment of reading the Sh'ma in the morning has passed, but the prescribed time for the blessings of the Sh'ma does not end until after four variable hours, as discussed later in Section 58. But if one is concerned that the congregation will also miss the prescribed time for the blessings of the Sh'ma, he should not wait for the congregation at all, since he will lose out on fulfilling the commandment of reciting the blessings of Sh'ma. He will also miss the prescribed time for the Silent Prayer (known as the Amidah or Shmoneh Esray, the central prayer of the morning service), which is also to be said ideally before the end of the fourth variable hour, as discussed later in Section 89. Rather, he should say the Sh'ma in its proper time together with its blessings, and recite the Silent Prayer on his own.
[Variable hours, or Sha'os Zmanios, are defined by dividing the period between sunrise and sunset, or dawn and the appearance of three stars, into twelve equal segments, each of which is a variable hour. The length of a variable hour therefore varies considerably between summer and winter, and one needs to consult a special time table applicable to one's locale to determine the last time on a given date for reciting Sh'ma and the Silent Prayer, among other things. -- LC] [Although I'm by no means certain about this, I believe the variation is roughly from 50 to 72 minutes from winter to summer. -- YM]
MB 33 - And one fulfills one's obligation with this - See in the Dagul Meirevavah and in the Novellae of Rabbi Akiva Eiger, who write that on a regular weekday one should not intend to fulfill one's obligation with this unless he has first put on Tefilin, because it says about one who reads the Shema [intending to fulfill the commandment] in the morning without Tefilin, that it is as if he is giving false testimony about himself. [Since the commandment of Tefilin is mentioned in the Sh'ma - "And you shall bind them..." - LC] But it is obvious that if it is impossible for him to put on Tefilin now for some reason, he should not let this prevent him from fulfilling the positive commendment of reading the Sh'ma, since Tefilin and reading the Sh'ma are two separate commandments, and one does not depend on the other. Where one is unable to put on Tefilin there is no prohibition of reading Sh'ma, since the statement (cited above) about giving false testimony against oneself applies only where one wilfully failed to put on Tefilin, and not where there is some compelling circumstance, as the Levush writes in Section 58 (see there). Also, see the Mishnah Brurah, Section 58, Subparagraph 5, where I have brought the words of the Levush. (After reciting the verses Sh'ma Yisrael and Baruch Shem Kavod following the morning blessings) one should say "You were the same before the world was created" and not "Before You created." [This begins the blessing that concludes "M'kadesh es Shimcha baRabim" - Who sanctify Your name among the multitudes, and the MB is specifying the wording of the beginning of this blessing - LC]