The Mitzva to Eat Matza and Maror
There is a Torah commandment to eat matza on the night of the 15th of Nisan, as it states: “In the evening, you shall eat matzot” (Shemot 12:18). This matza must have been guarded (shemura), as it states: “And you shall observe (u-shemartem) the matzot” (ibid. 17). Some are scrupulous about fulfilling the mitzva with matza that was made by hand for the sake of this mitzva (above 12:4). One who eats stolen matza does not fulfill his obligation (SA 454:4). Therefore, it is good to pay for the matza before Pesaḥ, or at least obtain the explicit consent of the storeowner to grant the buyer ownership of the matza even if it has not yet been paid for, because if the storeowner does not agree to give the matza on credit, one cannot fulfill his obligation with it (MB 454:15). As soon as one eats a kezayit of shemura matza he has fulfilled the Torah commandment, because all eating-related commandments in the Torah require the consumption of at least a kezayit.
The Sages ordained three additional kezeytim of matza to be eaten at the Seder, making a total of four. After reciting the berakhot of “ha-motzi” and “al akhilat matza,” we eat, le-khatḥila, two kezeytim: one from the top matza, for “ha-motzi,” and one from the broken middle matza for “al akhilat matza.” Later, we eat another kezayit with maror, for korekh, and at the end of the meal we eat one more kezayit as the afikoman (some say it is preferable to eat two kezeytim for the afikoman).
Before getting into the specifics of the size of a kezayit, let us clarify the practical halakha: There is a consensus that a kezayit is about a third of a piece of machine-made matza, and about the same-sized piece of a handmade matza. Thus, right after reciting “ha-motzi” and “al akhilat matza,” two-thirds of a machine matza must be eaten. Another third should be eaten for korekh and one more for the afikoman.
The kezayit of matza must be eaten continuously. If one pauses while eating, and as a result takes longer than a shi’ur akhilat pras to eat a kezayit, he does not fulfill the mitzva. We shall soon discuss exactly how much time a shi’ur akhilat pras is, but for now, it is enough to say that whoever eats a kezayit of matza continuously fulfills the mitzva without question and need not look at the clock, because the only way it is possible to take longer than akhilat pras is if one stops eating for a few minutes.