Ytdyklly

About a week ago, I saw in a siddur (might have been Koren) an instruction above the 1st Shema said as part of Birkot Hashachar stating that one should recite the 1st paragraph Ve'ahavta at this point of one feels that he will not be able to recite all 3 paragraphs before the time deadline for the morning Shema.

This had me thinking - does one fulfill the mitzvah of Shema by just reciting the first paragraph? Can he do this lechatchila? E.g. - he arises early in the morning, says the 1st paragraph and goes to sleep and when he awakens it is past the deadline?

Likewise for the night time Shema - he didn't say it earlier in the night - he awakens, says 1 paragraph, sleeps but when he awakens, it's daytime.

If, in fact, the mitzvah can be fulfilled by just 1 pargaraph, then whey do we need to say the other 2?

From the language of the Rambam (MT Kriat Shema 1:2) one sees he held the mitzva is to recite the three paragraphs. In a long list of "what if you made this mistake" in chapter 2, at no point does the Rambam considers that anything less than 3 paragraphs counts to fulfill the mitzva.

The Shulchan Aruch (OC 60:5) writes that one needs to have kavana (intention) during the first verse for the mitsva to be valid and, at a minimum, needs to read the three paragraphs.

The Rishonim debate whether the reading is a mitzva d'orayta or d'rabannan or a fulfillment of the mitzva to remember the exodus from Egypt (for the third paragraph). See here for a digest of sources.

The siddur means you should recite the first paragraph now, to fulfill the Torah-ordained part of the mitzvah, which has a strict time frame that is easy to miss. Then you will, of course, recite the full 3 paragraphs when you get to them, to fulfill the rest of the mitzvah. @mbloch already provided the sources.