(Mishnah - R. Yehudah): We check for Chametz at Ohr (the night of) Nisan 14, and (if not then,) on the morning of the 14th.

Pesachim 4a - Question: why do we check so early? We could check at the beginning of the sixth hour of the day. All agree that Chametz is not forbidden (mid'Oraisa) until the end of six hours!

Answer #1: We check early, for zealous people do Mitzvos right away.

Rejection: If so, it would suffice to check when morning comes!

(Beraisa): "Uva'Yom ha'Shmini Yimol" - one may circumcise any time during the day, but zealous people do Mitzvos right away - "va'Yashkem Avraham ba'Boker!"

Answer #2 (Rav Nachman bar Yitzchak): Chachamim fixed a time when people are at home. Also, light of a Ner is good for checking (at night more than it is during the day).

(Abaye): Therefore, even if a Chacham has a fixed time to learn at night, he should not begin learning on the night of the 14th (before checking), lest he learn at length and miss checking (in the proper time).

8a (Beraisa): We do not check by light of the sun or moon or a torch, only by light of a Ner, for it is good for checking.

Rishonim

Rambam (Hilchos Chametz 2:3): Mid'Rabanan, at the beginning of the night of Nisan 14 we check for Chametz using a Ner. This is because everyone is at home at night, and light of a Ner is good for checking. We do not fix to teach Torah at the time of the 13th. Similarly, a Chacham should not begin to read (written Torah) at this time, lest he learn at length and miss checking at the beginning of the time

Magid Mishneh: Tosfos learns from the Isur to learn at night before checking that one may not eat before checking. This is correct. All the more so, eating is forbidden!

Rosh (Pesachim 1:11): If an Achsadra (a room without four walls) gets much sunlight, one may check it during the day of 14th if he was distracted and did not check at night, like the Yerushalmi says. The Mishnah says that if one did not check at night, he checks on the morning of the 14th. This teaches that one must check with a Ner even during the day, even in a house with much light.

Ra'avad (Pesachim 1a Hasagos on Ba'al ha'Ma'or): The Mishnah said that we check at 'Ohr' of the 14th to teach that we begin close to the start of the night, when there is still some daylight, lest one be lazy or forget. Since there is some light, it is correct to call it 'Ohr', for a nice expression. If not for this, the Tana would not call darkness 'Ohr' for a nice expression.

Ran (1b DH v'Ika): Some permit checking early, on the 13th, as long as he uses light of a Ner. Even during the day one must use light of a Ner. The Beraisa says that we do not check using sunlight... we use only light of a Ner. This refers to one who transgressed and did not check on the night of the 14th, or he comes to check on the 13th. The Ba'al ha'Ma'or says so. This is wrong. The Yerushalmi connotes that one may not check on the 13th. The light of a Ner is good only at night.

Poskim

Shulchan Aruch (OC 431:1): At the beginning of the night of Nisan 14 we check for Chametz.

Darchei Moshe (1): The Beis Yosef (Siman 433) says that the Rosh, R. Yerucham, and Tur do not allow checking on the t13. We hold like them, unlike the Magid Mishneh. R. Yerucham says that even a dark house must be checked at night, for a Ner illuminates better at night than during the day. Even an idle man checks only at night.

Taz (1): We need both reasons. Chachamim needed to fix a clear time for the Bedikah, i.e. something changes then, and this alerts people to check. The Gemara suggested checking at six hours, for this is when Chametz becomes forbidden mid'Oraisa. It suggested checking when morning comes, for this is when Zerizim usually begin, like we find regarding Avraham. The only change at night is when it starts, for then people return home. This is usually Bein ha'Shemashos, when some time remains in the day, and we must wait until a Ner illuminates brightly, i.e. true night. While he waits, he is totally idle, lest he (begin an activity and) neglect checking in the proper time. Therefore, both reasons are needed.

Taz (2): Surely, one may not eat (before the Bedikah), for there is no quota of eating. One can be drawn to eat more, and he will delay a long time from when he returned home. Even if one normally has a fixed time to learn one chapter, and he can finish it before the Bedikah, perhaps he will investigate something and delay. The Bach asked why Chachamim forbade only eating and drinking before Kri'as Shma at night, but permitted learning beforehand. There, Chachamim were not adamant that one recite Shma shortly after returning home. They were concerned only lest one totally neglect the Mitzvah. It is common to fall asleep amidst eating, but not amidst learning. Here, we are concerned for delaying Bedikah from the proper time, which is the primary Mitzvah. R. Yerucham said that similarly, one may not learn before praying Ma'ariv with the Tzibur. The Gemara explicitly permits one who normally learns! The Gemara discusses one who learns a fixed amount at that time. He interrupts to say Shma. He is permitted, even though sometimes he delays an hour or two, since he has a fixed time to learn and to interrupt. R. Yerucham discusses one who has no fixed amount to learn. Now, he wants to learn, and there is no quota. R. Yerucham connotes that Ma'ariv is like Bedikah, i.e. for one who has no fixed amount to learn. One who has a quota may learn before Ma'ariv, but not before the Bedikah.

Mishnah Berurah (8): Those who pray Ma'ariv b'Tzibur should do so before the Bedikah, for it would be hard to gather people afterwards. Chak Yakov says that in every case one prays first, for Ma'ariv is more regular.

Mishnah Berurah (1): It is proper to begin checking before Tzeis ha'Kochavim, lest he be lazy or forget.

Kaf ha'Chayim (5): Eliyahu Rabah says that it begins at Tzeis ha'Kochavim. The Shulchan Aruch connotes like this. There is still a little light then. Chak Yakov, Chayei Adam, Kitzur Shulchan Aruch and Ben Ish Chai agree.

Mishnah Berurah (3): We require light of a Ner, for one must check in cracks, and a Ner is best at night.

Shulchan Aruch (2): One must be careful not to begin any Melachah or to eat before Bedikas Chametz. Even if he has a fixed time to learn, he does not learn until checking. If he began learning during the day, he need not interrupt.

Beis Yosef (DH v'Chasav): R. Yonah derived this from Abaye, who forbids starting to learn on the night of the 14th. If one who began learning during the day must interrupt, Abaye should have taught this (and all the more so, one may not begin to learn at night)!

Taz (3): I say that even R. Yonah permitted only one who has a fixed amount to learn. Since the only concern is delaying the Bedikah, Chachamim did not decree. If he has no fixed amount, he agrees that it is forbidden, just like regarding Ma'ariv. Surely even this opinion requires him to interrupt eating, for it has no limit.

Magen Avraham (3): One may not begin half an hour before dark, just like we forbid half an hour before the time for Minchah and Ma'ariv. One who did not check at night is forbidden (Melachah and eating) before checking during the day.

Magen Avraham (5): People who say a Halachah in the Beis ha'Keneses after Tefilah are permitted. We forbid only learning in one's house, for he need not get up. One who is in the Beis ha'Keneses will surely go home. However, one who learns in depth is forbidden even in the Beis ha'Keneses. We decree lest this extend, and he will forget to check afterwards. We forbid also entering a bathhouse and everything forbidden before Minchah.

Kaf ha'Chayim (16): The Pri Chodosh forbids all learning after the time for Bedikah.

Bach (4 DH Pasak): Machzorim say to check before Ma'ariv. I say that he should pray first, lest he be distracted checking and forget to pray. Tefilah is fixed; it will not cause him to forget to check. One who always prays should check first. Since he is used to this, he will not forget. R. Yerucham similarly says that if one normally prays in the Beis ha'Keneses, and now forgot, he may not learn before praying. He permits one who always prays Ma'ariv at home.

Rebuttal (Magen Avraham 5): R. Yerucham concludes that even one who always prays Ma'ariv at home may not learn before praying. The Shulchan Aruch (89:6) rules like this. The time to check is before Tzeis ha'Kochavim. He prays afterwards, after Tzeis. If he did not check before Tzeis, he tells someone else to check for him, and he prays. If no one else can check for him, he does like the Bach says.

Kaf ha'Chayim (11): Chak Yakov says that we are not stringent to annul learning before the Bedikah. Chayei Adam is stringent only Bein ha'Shemashos, but not half an hour before the Bedikah. Chemed Moshe says that all the Acharonim forbid half an hour before the Bedikah.

Taz (4): Maharil permits merely tasting food. Only a meal is forbidden.

Mishnah Berurah (6): Tasting is any amount of Peros, or a k'Beitzah of bread, like we say about eating before Minchah. One may eat much Peros only in the half hour before the Bedikah. Once the time came, it is improper to delay a lot.

Kaf ha'Chayim (15): The Magen Avraham permits drinking no more than k'Beitzah (of wine). There is more concern lest he get drunk (and not check) than for eating (this amount). A k'Beitzah is the Shi'ur for anything from the five grains, just like for bread.

Magen Avraham (6): The Shulchan Aruch connotes that all agree that one must interrupt an activity that is not a Mitzvah.

Magen Avraham (7): If one started at night, all agree that he interrupts, for before Bitul, Bedikah is mid'Oraisa, and one must interrupt for a Torah Mitzvah.

Mishnah Berurah (7): Some forbid learning half an hour before the Bedikah unless he asked someone to remind him when the time comes. Others permit until Tzeis ha'Kochavim.

Mishnah Berurah (9): Likewise, one need not interrupt other labors if he started during the day.

Kaf ha'Chayim (14): If one cannot check now, e.g. he lost the key and is waiting for a locksmith to open the door, he must be idle until he comes. If he will not be able to check at night, everything is permitted.

Rema: Some say that he must interrupt. This seems primary.

Beis Yosef (DH u'Mah she'Hiksheh): The Tur said that since the concern is lest he learn at length and forget to check, it does not matter when he started. I say that since this matter is mid'Rabanan, it suffices to forbid learning at night. We need not be so stringent to stop one who started during the day.

Taz (4): The Tur holds that surely one who started during the day must interrupt, for when he began he did not think about the Mitzvah. Perhaps he will not think to interrupt to check (and he will neglect the Mitzvah)! The Gemara teaches that even if he began after dark, and knew that he must check, and begins to learn with this intent, even so it is forbidden lest he delay a lot from when he returned home.

Bach (5): Regarding Ma'ariv, if one began while it was permitted, he need not interrupt. Here we are concerned lest he forget, since Bedikah is only once a year.

Rebuttal (Magen Avraham 8): Lulav is once a year, yet one who began while it was permitted need not interrupt! Chidushei Hagahos ha'Tur (3) says that Bedikah is more stringent, for it is to prevent Isur. I say that 'if he started during the day' means a half-hour before night, when it was forbidden to start. The Beis Yosef says that he need not stop because it is not mid'Oraisa, for Chametz is not forbidden yet.

Kaf ha'Chayim (21): The Acharonim explain that the Shulchan Aruch discusses one who began when it was permitted.

Mishnah Berurah (12): One must interrupt also other Melachos, i.e. when Tzeis ha'Kochavim comes.

Kaf ha'Chayim (23): Chayei Adam says that one who began learning b'Heter may continue after dark, but not one who began eating b'Heter.