Question: In the Summer, I typically daven Shacharis too late to recite the Shema before it is too late. Do I need to say all three paragraphs before Shacharis, or just the first line?
Answer: The Mishna (Berachos 9b) teaches that there is a machlokes as to when one must recite the Shema by. The Gemara (Berachos 10b) teaches that the halacha follows R’ Yehoshua who says that it must be recited before the ‘third hour’. There is a machlokes as to how to calculate this time.
The Rema (OC 46:9) writes that it is ideal to recite the first line of the Shema followed by ‘baruch shem’ at the beginning of davening so as not to miss the latest time to recite the Shema.
One is obligated to recite the first two paragraphs mideoraisa while the third paragraph contains mention of yetzias mitzrayim. The Shulchan Aruch Harav (OC 46:9) notes that there are other places in pesukei dezimra where we mention yetzias mitzrayim. Nonetheless, the Mishna Berura (46:31) quotes the Pri Chadash (OC 46:9) and Chayei Adam (1:8:7) who write that one should ideally recite all three paragraphs.
R’ Avraham Yeshaya Pfoifer (Ishei Yisrael 18:17) writes that ideally one should recite all three paragraphs. If necessary, one could just recite the first paragraph. One who was really pressed for time could just say the first line of the Shema followed by ‘baruch shem’.
In conclusion, if one thinks they may not be able to recite the Shema in Shacharis before sof zeman kerias Shema, they should ideally recite all three paragraphs during the korbanos or before Shacharis.