Contributed by: R. Ezra Mizrahi
As we prepare for Pesah, we must always revisit the Haggadah and understand what comprises the seder, a tradition that was performed by the Jewish people since the exodus from Egypt, over three thousand years ago.
Towards the end of Magid, the part of the Haggadah where the entire story of the Exodus from Egypt is recited aloud, the first two paragraphs of the Halel are said. Then there is the blessing of "Gaal Yisrael". Afterwards, the Massah and Maror are eaten, and of course the full meal. In part two of the seder, after Birkat Hamazon, the remainder of the Halel is said. This is the only time in the year that we break up the Halel into two different parts.
Why do we break the Halel into two parts during the seder, for wouldn't one expect to say the Halel as always, in its entirety?
The first two paragraphs in the Halel, if you pay attention, speak specifically about the Exodus from Egypt. This fits with the first part of the seder, the ending of the story of the Jewish nation's servitude there. We end with a blessing of praise to Hashem, "Gaal Yisrael," the Redeemer of the nation of Yisrael. The remainder of the Halel, deals with general praise and turns towards a future redemption- the ultimate redemption. Therefore, we begin the second part of the seder (after Birkat Hamazon) with "Lo Lanu" and the continuation of the Halel. However, because such a phrase is not fitting to begin with ("lo lanu" literally means "not to us"), we begin with "Shefoch Hamatecha etc."