(Mishnah): On the night of the 14th of Nisan we check for the Chametz to the light of a candle (in order to avoid transgressing bal yeroeh u'bal yemotzai - having Chametz in your possession on Pesach).

One need not check any place to which one does not take Chametz.

Question: In which case, why does the Mishnah (see further) require checking two rows in a wine cellar?

Answer: It refers to when one takes Chametz there.

Beis Shammai say that one must check two rows across the whole face of the cellar (this will be explained);

Beis Hillel say the two outermost (which are the top two) rows.

(Gemara): The pasuk says, (Shemos 12, 17-18) "And you shall guard the matzos.....On the first day, on the 14th day of the month in the evening you shall eat matzos etc."

What is "on the first day... you shall eat matzos" referring to? If it refers to eating matzah - the pasuk already says, (Pasuk 15) "shivas yomim matzos tocheilu" - "seven days you shall eat matzos";

If it is to teach that those seven days begin on the 14th - the pasuk says "until the 21st day of the month" - so the seven days must begin on the 15th.

Rather, if it does not refer to eating matzah, it must refer to destroying Chametz.

Question: Why should one check by candlelight?

Answer (Rav Shmuel bar Rav Yitzchak): Since a candle checks everywhere (even in nooks and crannies).

Question: Why check at night? (in order to fulfill the pasuk, anytime on the 14th would suffice)

Answer #1 (Rav Yosa): Checking by candlelight is only beneficial at night.

Objection (Rebbi Mana): This is not the reason - rather its source is Scriptural -

Answer #2: "And you shall guard ("Ushemartem") this day for your generations as an eternal law" -ensure that both the night and the day of the 14th are guarded from Chametz by checking for it on night of the 14th.

Question: Perhaps one should check on the day of the 13th so that the entire night of the 14th will be guarded from Chametz?

Answer: RebbiMana also agrees to Rebbi Yosa that candlelight is only beneficial at night.

Question: Then perhaps one should check on the night of the 13th?

Answer: If so, one could also check on Rosh Chodesh [Nisan]! (But the Torah only requires the 14th to be guarded but not before).