Sanhedrin, Chapter 7, Mishnah 4. After listing the four methods by which the court administers capital punishment, the Mishnah proceeds to detail, over the course of several chapters, the various transgressions that incur the death penalty. Our Mishnah lists the eighteen transgressions punishable by stoning - provided, of course, that all the prerequisites were met: witnesses, forewarning, and all the other details.
In total, the Mishnah enumerates thirty-six prohibitions punishable by death, and they are divided as follows:
Stoning - eighteen transgressions.
Burning - ten transgressions.
Decapitation (beheading) - two transgressions.
Strangulation - six transgressions.
Eighteen plus ten, plus two, plus six - altogether thirty-six.
These are the eighteen transgressions punishable by stoning:
"Haba al ha'em" - One who has relations with his mother. She does not necessarily have to be his father's wife; even a woman who was never married to his father but gave birth to him is included in this.
"Ve'eshes av" - A woman married to his father, or who was married to him in the past: his stepmother, and even his former stepmother. Later on, the Mishnah will discuss the case where she is both his mother and his father's wife, as in a regular family, and there it will be seen that there is room for dual liability.
"Ve'al hakallah" - His daughter-in-law. A woman who was married to his son is forbidden to him forever.
"Haba al hazachur" - A man who lies with a man in the same manner as one lies with a woman.
"Haba al habehemah" - A man who lies with an animal in the manner of a woman.
"Veha'ishah hamevi'ah es habehemah" - The same law applies to a woman who lies with an animal in the manner of her husband.
"Hamegadef" - One who curses Hashem. The details of this law will be explained in an entire Mishnah immediately following this one, with Hashem's help.
"Veha'oved avodah zarah" - One who worships idols and false gods; this matter will also be described in detail in another Mishnah.
"Vehanosen mizar'o lamolech" - One who passes his children over and through the fire in the Canaanite ritual called Molech; this will be discussed in half a Mishnah coming shortly.
"Uva'al ov" - One who engages in the fortune-telling technique called Ov.
"Veyid'oni" - A separate matter in its own right, and it too will be explained in half a Mishnah coming shortly.
"Vehamchalel es haShabbos" - One who desecrates the Sabbath by performing one of the thirty-nine melachos prohibited by the Torah. This is not discussed here.
"Vehamkalel aviv ve'imo" - The same law applies to both parents, the father and the mother, and this refers to one who mentions the Name of Hashem when cursing them.
"Vehaba al na'arah hame'orasah" - A 'na'arah' is a young girl who has passed the age of bas mitzvah and is still within the six months following it, meaning between twelve and twelve and a half. 'Me'orasah' means that she accepted kiddushin from her husband and is married from a halachic standpoint, but is not actually married: she has never lived with him and is still in her parents' home. The Torah views this with particular severity, as she is young, in the prime of her life, and was never actually married. This transgression is listed separately from the previous six forbidden relationships, most likely because those six are listed in the Torah portions of Acharei Mos and Kedoshim, whereas this one appears entirely separately, in the portion of Ki Seitzei.
"Vehamesis" - One who leads a single Jew astray from Hashem in order to worship idols.
"Vehamadiach" - The very same concept, except that he does not lead an individual astray but rather an entire city - which is the 'ir hanidachas' (subverted city), from the same root word - to idol worship.
"Vehamchashef" - One who engages in sorcery and magic. This too is prohibited by the Torah, and carries the severe punishment of stoning.
"Uven sorer umoreh" - A wayward and rebellious son who meets a series of criteria known to many: he is a glutton and a drunkard, does not listen to his parents' voices, and more. The eighth chapter, which follows immediately, is mostly dedicated to expanding upon the laws of the wayward and rebellious son, and it will be examined there.
To summarize the tally: seven transgressions of forbidden relationships; eight transgressions of betrayal against Hashem - the blasphemer, the idol worshipper, one who gives of his offspring to Molech, the Ov and Yid'oni practitioners, the instigator of an individual, the instigator of a city, and the sorcerer, making a total of fifteen; two transgressions of severe offense to the honor of parents - one who curses his parents and a wayward and rebellious son; and finally, the Sabbath desecrator.
One Transgression - Two Prohibitions:
From here the Mishnah moves on to discuss a case where a person committed two of these transgressions simultaneously, and the fundamental answer is that he is punished twice. It goes without saying that one who is executed, Heaven forbid, has only one life, and there is no possibility of a second execution. Rather, in all these cases, when the conditions for execution were not met and the person acted willfully, the consequence is kares - being cut off by Heaven, a premature death. And if he transgressed unintentionally, without meaning to, not knowing that the act was forbidden - he thought he was marrying an ordinary woman and it turned out to be his mother, or he did not know that today was the Sabbath and desecrated it - he is obligated to bring a sin-offering. It turns out that the double punishment under discussion is not a double death penalty, but rather the obligation of two separate sin-offerings, which atone for two transgressions committed as one.
The Mishnah brings several examples of this, which include a dispute. The first: "Haba al ha'em chayav aleha mishum em umishum eshes av" - One who has relations with his mother is liable for it on account of her being a mother and on account of her being a father's wife. If one has relations with his mother and she is married to his father, there are two separate prohibitions here: the first - on account of a mother, for this is his mother, and if he did this unintentionally he is liable for one sin-offering; and the second - on account of a father's wife, for this woman is married to his father, and any woman who married his father is forbidden to him forever, whether the father is still married to her or is no longer alive. In this case he is liable for two sin-offerings, as there are two separate and distinct prohibitions present.
Rabbi Yehudah disagrees and holds that he only brings one sin-offering, on account of his mother alone. He bases this on the verse "The nakedness of your father and the nakedness of your mother you shall not uncover... she is your mother": 'The nakedness of your father' refers to your father's wife, who is your mother, and 'you shall not uncover' - meaning you are forbidden to lie with her; and since the verse concludes "she is your mother," it is derived that even though she is both your father's wife and your mother, she is considered only as a mother, one transgression. The Sages disagree with this exegesis, for in their opinion 'the nakedness of your father' refers to the father himself: a separate prohibition to have relations with your mother and a separate prohibition to have relations with your father, and both are forbidden.
The second case: One who has relations with his father's wife while she is still married to his father, meaning his stepmother. Here too there are two liabilities: one on account of a father's wife, and the second on account of an ordinary married woman, which is a separate prohibition in its own right (and its punishment is indeed not stoning but strangulation, but that is beside the point). Regarding the sacrifices, he must bring two sin-offerings. It is worth noting that in the previous case, when the woman is his mother and she is still married to his father, he is essentially checking three boxes at once.
The prohibition of a father's wife applies whether during the father's lifetime or after his death: his stepmother, his late father's wife, remains forever forbidden with this punishment. And there is no difference whether the marriage was only erusin from a halachic standpoint - that he gave her a ring and they never lived together - or an actual complete marriage.
Similarly regarding one who cohabits with his daughter-in-law: first, there is the prohibition of a daughter-in-law, and if she is still married to his son, she is also a married woman - a double prohibition and two sins. And this applies whether during the son's lifetime or after his death, whether the marriage was fully actualized and they lived as husband and wife, or whether it was only erusin, where she merely received the value of a perutah from him.
To illustrate this point: imagine a woman who has no connection to Avram and his son. They negotiate between themselves as to which of them will marry her, and they decide by tossing a coin, and the son wins. The son gives her the coin and she accepts it, and she is betrothed to him. A moment later, the son is struck by lightning and dies. From now on, the father can never marry this woman, even though five seconds earlier he was permitted to marry her. All that happened is that she received a coin from the son, and the son died - and once she was betrothed to the son, she is forbidden to the father forever with the punishment of stoning.
A note regarding the forbidden relationships listed above: one who cohabits with a woman who is forbidden to him with these punishments - assuming she consented to the act - she too receives the exact same punishment as he does. If she did not consent and he raped her, she certainly is not punished. The rule is that both partners are punished when they acted intentionally, and when both acted inadvertently - both bring a sin offering.
Stoning the Animal:
The Mishnah repeats and mentions here "habo al hazachur" - one who cohabits with a male, "habo al habehemah" - one who cohabits with an animal, and a woman who lies with an animal. The main novelty is in the latter two: not only is the person who lay with the animal executed, but the animal itself is stoned.
This deeply troubles the Mishnah, since the animal is not commanded in mitzvos and has no free will, so what can it be blamed for? Therefore, the Mishnah asks: "im adam chata, behemah mah chata'ah?" - assuming that the person committed a severe sin here, what did the animal do? Why is it killed?
There are two answers to this:
"ela lefi sheba'ah leadam takalah al yadah, lefichach amar hakatuv tisakel" - since this animal was the catalyst that brought ruin upon a human being, and a person stumbled because of it, the Torah decreed that it be stoned, so that it does not happen again with another person.
"davar acher: shelo tehei behemah overes bashuk veyomeru zo hi sheniskal peloni al yadah" - this is out of respect for the person who was executed. If that animal were to walk through the marketplace, onlookers would point at it and say: Here is the animal because of which so-and-so was stoned. This would remind people of him, his downfall, and his shameful situation; and to prevent this, for the sake of his honor, the animal is done away with as well.