The difference between a request and a command
Many times, the Torah uses the word לא, lo (don’t), when it wants to express a negative statement. However, other times it uses the word אל, al (don’t). An example of both is in this week’s parsha, in two adjacent verses. The Torah introduces the mitzvah of the korban Pesach, the Passover offering, with a list of several instructions for its preparation and consumption. All of these instructions constitute individual mitzvos. There’s a mitzvah not to eat a korbon Pesach which wasn’t roasted properly over a fire. Regarding this mitzvah, the Torah uses the word al, when it says not to eat from it insufficiently roasted. There’s another mitzvah not to leave any of the korbon Pesach over until morning. It must be entirely consumed. With this mitzvah, the Torah uses the word lo, when it says don’t leave any of it over. What’s the difference between these two words? Why does the Torah sometimes choose one over the other?
Rabbi Meir Simcha HaCohen of Dvinsk understands that the word al is used when the Torah wants to express a request. While the mitzvah is still obligatory, the tone is different. Therefore, the word נא, please, can only accompany the word al. You’ll never see the word “please” when the Torah uses the word lo. The word lo is more indicative of a command. But this doesn’t fully answer the question. Why do some mitzvos use a word indicating a request, whereas others use a word which is more forceful?
Before Hashem annihilated all the firstborns in Egypt, it was clear that the Egyptians still felt Moshe was behind all of the plagues. Before the plague of locusts struck, the seventh of the ten plagues, the Egyptians asked Pharaoh: “How much longer will [you allow] this to be a menace to us?” They were speaking about Moshe, as if he was the cause of all their suffering. They didn’t realize yet that everything was from Hashem. However, once Hashem struck the firstborns, they all realized that there was special Divine intervention for every individual. In fact, this was the entire purpose of the ten plagues. Hashem wanted everyone to realize that he has intimidate involvement in every aspect of everyone’s lives.
This overseeing is alluded to with the mention of the Exodus from Egypt juxtaposed to many mitzvos throughout the Torah. For example, the Exodus is mentioned in the prohibition against usury, the mitzvah of tzitzis, and the mitzvah to keep only honest weight and measures. At the culmination of the Exodus, Hashem discerned between who was a firstborn and who wasn’t. The child which resulted from an Egyptian woman’s unfaithfulness, even though this child only looked like her second, he also died. He was in fact the firstborn of the man she cheated with. Nothing escaped Hashem’s watch. So too, Hashem tells us that He is able to discern those who lend with interest, but give excuses to appear as if it’s permissible. He can discern those who use fake dies for their tzitzis and say its kosher. Finally, He can discern those who try to hide from their customers that they are using dishonest weights.
Only once the Jews realized that Hashem is involved in every aspect of their lives, could their relationship truly blossom. Before the plague of the firstborns, this reality hadn’t been entirely evident. The Egyptians still thought Moshe was behind all of the plagues. Perhaps some Jews did too. Therefore, when Hashem commanded the Jews to prepare the korban Pesach, it would be premature to use the command form of lo. Instead, He commanded them in the form of a request, saying al. Please don’t eat from it insufficiently roasted. The Jews ate the korban Pesach the night of the Seder, and when midnight struck, all the firstborn Egyptians died. After that moment, it became clear that Hashem oversaw everything. Hashem had utter knowledge of all events, and was taking care of the Jews at the deepest possible level. Therefore, the mitzvah not to leave any leftovers from the offering until morning, could be given as a command, using lo. That mitzvah applied after the death of the firstborns. That was when their relationship had truly begun to take shape.
 Based on Meshech Chochmah to Exodus 12:9-10
 Rashi ad. loc.; cf. Ibn Ezra who translates it as raw, and Rashbam who says it means roasted in a pot, without water (as opposed to an open flame)
 Exodus loc. cit.
 Regarding other examples, see note 19
 Sefer HaMitzvos Lo Sa’aseh § 125, based on our verse. Cf. Ramban ad. loc. and Semag Lavin § 350, 351 who count this verse as two mitzvos: 1) not to eat it נא 2) not to eat it if it was cooked in water
 The Sefer HaChinuch § 7 explains the reason for this mitzvah, as with all of the Passover mitzvos, is to have us remember the Exodus from Egypt. By eating the korban Pesach roasted, like dignitaries who enjoy delicacies of meat, we remind ourselves that we were taken out from slavery to become a majestic, free people
 Sefer HaMitzvos Lo Sa’aseh § 117; Semag Lavin § 358
 The Sefer HaChinuch § 8 explains, like in note 5, that we need to eat the korban Pesach like dignitaries. Aristocrats don’t leave over any of their food, as they aren’t worried about where tomorrow’s meal will come from. Only poor people need to ensure that they have leftovers, incase they can’t find a meal the next day. That’s why if any of the offering is left over, it needs to be burned. This shows we don’t have any use for the leftovers, like a dignitary who would throw theirs out
 The Meshech Chochmah didn’t write this, but I assume he felt it was self-understood, as mitzvos aren’t optional
 For example: Genesis 18:3, 18:30
 Translation from The Living Torah by Rabbi Aryeh Kaplan
 Exodus 10:7
 Bava Metzia 61b
 Leviticus 25:37,38
 Numbers 15:39-41
 Leviticus 19:35,36
 Rashi to Exodus 12:30, his source being Tanchuma Yashan Bo § 19
 Rashi to Bava Metzia loc. cit.
 The Meshech Chochmah proceeds to bring other examples of this principle as it appears throughout Chumash. Rav Kupperman points out that the examples he brings ends up showing that there are three cases where the word אל is used instead of לא: 1) When the desire of the Torah is to command something, but for one reason or another Hashem didn’t want to make it a full-force command. He sees this from Leviticus 25:36, which sounds like it’s prohibiting lending to a ger toshav with interest (see Bava Metzia 70b). 2) When the prohibition is so self-evident it doesn’t need to be written in a strong form. An example is ibid 25:14, the prohibition to overcharge another. 3) Sometimes it is simply not feasible to have a mitzvah stated in the form of a definite command. An example is ibid 19:4, which makes a prohibition on a person’s thoughts, which are hard to control