It is forbidden to derive benefit from leaven (chametz) or leavened food on Pesach, as it says "Leaven shall not be eaten" -- "it shall not be permitted in any way that leads to eating".¹ One who keeps leaven in his possession on Pesach even if he does not eat it violates "No leavening shall be seen in all your border"² and "Leavening shall not be found in your houses"^3,a. It is also forbidden to derive benefit from leaven after noon of the day before Pesach, as it says "You shall not eat leaven with it (i.e., with the Pesach sacrifice)"⁴; the sages extended this prohibition to the hour before noon and forbade eating leaven beginning two hours before noon.^b It is rabbinically forbidden to derive benefit after Pesach from leaven which was in the possession of a Jew during Pesach.^c If leaven becomes mixed with other things it is still forbidden to eat it on Pesach, as it says "You shall not eat anything leavened"⁵; but after Pesach it is permitted.^d

We are commanded to dispose of leaven before it becomes forbidden to eat it, as it says "On the first day you shall dispose of leavening from your houses"⁶; "first day" here means the day before Pesach, as it says "You shall not slaughter My sacrifice on leaven" -- "you shall not slaughter the Pesach sacrifice while leaven still exists".⁷ The disposal to which the Torah refers consists of annulling it in one's mind and regarding it as dust; rabbinically one must also search for it and remove it from one's premises. The search is done by candlelight on the evening of the day before Pesach^e; the leaven found must be destroyed by an hour before noon the following morning. If the day before Pesach is the sabbath the search is conducted a day earlier, and only enough leaven to eat on the morning of the sabbath is kept.^f One may also sell his leaven to a non-Jew and buy it back after Pesach.^g

Forbidden leaven is that made from five types of grain (wheat, spelt, barley, rye, oats).^h When making unleavened bread for Pesach (see below) one must be careful not to let it become leaven, as it says "And you shall guard the unleavened bread".^8,i

b) Unleavened Bread

We are commanded to eat on the first evening of Pesach unleavened bread (matzah) made from one of the five types of grain, as it says "In the evening you shall eat unleavened bread".^9,j It is rabbinically forbidden to eat matzah on the day before Pesach, to eat heavily during the latter part of that day, and to eat anything after the last piece of matzah in the evening.^k The Pesach sacrifice was eaten with both matzah and bitter herbs (maror); the sages instituted that maror as well as matzah be eaten on that evening even in the absence of the Pesach sacrifice, and they also instituted dipping the maror in charoses, a mixture of fruits resembling mortar.^l

On that same evening we are commanded to recount the miracles and marvels that were performed for our ancestors in Egypt, as it says "Remember this day when you went out of Egypt",¹⁰ and it says "And you shall tell your son on that day: `Because of this'" -- "at a time when matzah and maror lie before you".¹¹ In particular one must mention [i.e., explain the significance of] three things: the Pesach sacrifice, matzah, and maror.^m

In every generation one should regard himself as though he personally had come out of the Egyptian bondage, as it says "And He took us out of there"¹²; concerning this the Torah commands "And you shall remember that you were a slave [in Egypt]".¹³ One should therefore eat and drink that evening in a reclining position, like a free man, and everyone is required to drink four cups of wine.ⁿ

Sources:
1. Ex. 13:3 and Pesachim 21b	a. 1:2
2. Ex. 13:7, Deut. 16:4	b. 1:8-9
3. Ex. 12:19	c. 1:4
4. Deut. 16:3	d. 1:5-6
5. Ex. 12:20	e. 2:1-3
6. Ex. 12:15	f. 3:1,3
7. Ex. 34:25 and Mechilta on Ex. 23:18	g. 4:6
8. Ex. 12:17	h. 5:1
9. Ex. 12:18	i. 5:9
10. Ex. 13:3	j. 6:1,4
11. Ex. 13:8 and Mechilta on it	k. 6:11-12
12. Deut. 6:23	l. 7:11-12; 8:8
13. Deut. 5:15; 15:15; 16:12; 24:18,22	m. 7:1,5l. 7:11-12; 8:8
	n. 7:6-7