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The prohibition against eating Chametz:
The general prohibition:
It is Biblically forbidden to eat any amount of Chametz [even if the Chametz is ownerless or is owned by a gentile] and one who does so, under certain conditions, is liable for the penalty of lashes and Kareis.
Taaruvos Chametz-Chametz mixtures: Mixtures of food that contain Chametz, at times contain a Biblical eating prohibition, at times a Rabbinical eating prohibition, and at times is permitted to be eaten, as explained in Halacha 1H!
Spoiled, inedible, Chametz: Chametz that became inedible even for dogs, before the 6th hour on Erev Pesach, is Rabbinically forbidden to be intentionally eaten, as explained in Halacha 1G!
Eating Matzah Ashira; Chametz Nuksha; etc: See Halacha 1E-G!
Irregular form of eating: Biblically, it is only forbidden to benefit from Chametz in the regular fashion of benefit. However, Rabbinically, even irregular benefit of Chametz is forbidden. Nonetheless, if one needs to eat Chametz for medicinal purposes, the Sages permitted irregular eating to be done if one does not own the Chametz, such as he borrows it from a gentile. This applies even if the illness does not involve danger. [Accordingly, from the letter of the law, it is permitted to swallow medicine capsules on Pesach even if they contain a Chametz ingredient, as explained in Chapter 6 Halacha 5.]
Eating Chametz for medical reasons or if one has nothing else to eat such as a child: It is permitted [and an obligation] to eat Chametz for the sake of saving a life. Accordingly, one who is dangerously ill may eat Chametz if that is the only option available. Likewise, a child may eat Chametz for medical reasons. This matter will be discussed in chapter 6 Halacha 6-See there!
When does it become prohibited to eat Chametz?
It is Biblically forbidden to eat Chametz starting from midday [the start of the 7th hour of the day] of the 14th and onwards, throughout the seven days of Pesach, and one who does so transgresses a negative command. The Sages added two more Zmaniyos hours towards this prohibition, and it is thus Rabbinically forbidden to eat Chametz from the beginning of the 5th hour from sunrise and onwards. [Practically, this is equivalent to the time of Sof Zman Tefilah for Shacharis.] Starting from this time, it is forbidden to eat even Rabbinically forbidden forms of Chametz [i.e. Chametz Nuksha; Rabbinical Taaruvos Chametz].
 Admur 442:7; 28; Kuntrus Achron 442:17; 446:1; Rambam Chametz 1:7; Yuma 73b
The reason: It is forbidden to eat any amount of Chametz, even less than a Kezayis, and even if the Chametz has mixed into another food and only gives taste, as Chetzi Shiur is Biblically forbidden by all food prohibitions mentioned in the Torah. This is learned from the verse which states “Kol Cheilev,” which is coming to include even Chetzi Shiur [less than the standard measurement]. [Admur 442:28]
 When does the Kareis and lashes penalty apply for one who eats Chametz? One only receives the penalty of lashes for eating Chametz if all the following conditions are fulfilled: 1) One eats a Kezayis of the Chametz. 2) One eats the Kezayis within the amount of time it takes to eat a pras of bread [between 4-11 minutes]. [See Admur 442:1] 3) One eats the Chametz Bemeizid. [Rambam 1:1] If the above conditions are fulfilled one also receives the penalty of Kareis, if he ate the Chametz beginning from the night of the 15th and onwards. [Rambam 1:1 and 8] Regarding the penalty of Kareis/lashes for eating Taaruvos Chametz: See Halacha 1H in footnotes!
 Rambam Yesodei Hatorah 5:8; See Admur 466:4 regarding benefit
Definition of irregular form of eating: See Rambam ibid that eating it together with a bitter substance which removes the benefit of the taste demotes it from being a Biblical prohibition.
 Admur 450:24-26; 466:5 regarding owning and benefit; Rambam Yesodei Hatorah 5:6;
 Rabbinical Chametz: It is prohibited to eat Rabbinical Chametz, beginning from the same time that it is forbidden to eat or benefit from Biblical Chametz. Thus, it is forbidden to eat or benefit from starting from the Biblical time of midday. [Olas Shabbos 443:1; Chok Yosef 443:2; Kaf Hachaim 443:4] Furthermore, due to the Rabbinical decree explained next, it is Rabbinically forbidden to eat even Rabbinical Chametz starting from the 5th hour.
 Admur 431:2
The source for prohibiting eating from midday: As the verse states “You shall not eat upon it Chametz”, [“it” here] refers to the Pesach sacrifice. Now, the verse is not coming to warn against eating Chametz at the time that one eats the Pesach sacrifice, which is on the night of the 15th, as there is a verse which already states this by saying “do not eat Chametz…”. Rather, the verse is coming to warn against eating Chametz at the time of the sacrifice of the Pesach, which is from midday of the 14th and onwards. [Admur ibid]
 How to calculate the hours: Shaos Zmaniyios is calculated by dividing the number of minutes of daylight by 12. Thus, the two hours are calculated in accordance to the number of minutes that exist within the 12 hours of daylight within one’s area. Thus, [both in areas where the daylight hours are less than 60 minutes per hour, and] in places where the daylight hours is more than 60 minutes per hour, one follows the Zmaniyos amount of minutes that exist within the hour, and not 60 hour minutes. [Admur 443:4; M”A 443:3; Olas Shabbos 443:1; Chok Yosef 443:7; Chayeh Adam 123:3] For example, if there are 15 hours of daylight in ones area [900 minutes], then each hour contains 75 minutes. The reason behind this calculation is because the daytime which is between sunrise and sunset is always considered to be twelve hours whether the day is long or short. Thus, in the above example one must stop eating Chametz starting from 2 hours and 30 minutes before midday, and must sell the Chametz to the gentile by 1 hour and 15 minutes before midday.
Other opinions in Admur: Some Poskim rule that these hours are [no more than] 60 minutes each, as is the case by average days [as since these two hours are only Rabbinical therefore one may be lenient]. [brought in Admur 443:4]
 Admur 443:4; 459:10; Siddur in Hilchos Kerias Shema; GR”A; Chok Yaakov and other Poskim; Vetzaruch Iyun from Admur 89:1 regarding Davening
Other opinions: Some Poskim rule we calculate the hours starting from Alos Hashachar. [M”B 443:8 in name of many Poskim; Admur 89:1 regarding Davening; M”A 233:3; Peri Chadash 89; Levush 233, 267]
 Admur 443:1; Michaber 443:1
The reason: Although Biblically the prohibition to eat and own Chametz only begins from the end of the 6th hour, nevertheless the sages added two more hours towards this prohibition in order to prevent one from transgressing if he gets mixed up on a cloudy day and comes to think that midday is really two hours latter then what it really is. [Admur ibid] The Sages did not prohibit eating Chametz prior to the 5th hour [during the 4th hour] as one will not come to make a mistake in this matter as this is the time of eating for all people [and they thus know the time]. [M”A 443:2; Kaf Hachaim 443:6]
 Admur 443:1; Chok Yaakov 443:3; Chemed Moshe 443:1; M”B 433:3; Kaf Hachaim 443:7
Other Opinions: Some Poskim rule that one may eat Chametz Nuksha during the 5th hour, until the 6th hour of the day. [Noda Beyehuda 21, brought in M”B and Kaf Hachaim ibid; See Olas Shabbos 443:1; Chok Yosef 443:2; Kaf Hachaim 443:4]