1. There is a dispute in the Mishnah about whether a Sukah that is 20 Amos tall is valid.
2. The Mishnah cites three other laws of a Sukah, including the law that a Sukah must have three walls.
3. The Gemara teaches three opinions for why the Tana Kama rules that a Sukah that is 20 Amos tall is invalid.
4. Three opinions limit the dispute in the Mishnah (#1) to specific cases.
5. The Gemara explains that these three opinions (#4) were said only according to the understanding (#3) of the Tana Kama.
A BIT MORE
1. Tana Kama: It is not valid. Rebbi Yehudah: It is valid.
2. The two other laws are that the Sechach must provide more shade than sun, and the walls of the Sukah must be higher than ten Tefachim.
3. Rabah: One does not even realize he is sitting under Sechach when the Sukah is 20 Amos tall. Rebbi Zeira: A person is not sitting in the shade of the Sechach but rather in the shade of the walls. Rava: A Sukah is supposed to be a temporary structure, and one cannot build a structure 20 Amos tall without it being a permanent structure.
4. For example, Rav Huna says that the dispute in the Mishnah applies to a Sukah that is only four by four Amos. If it is wider, everyone agrees that the Sukah is valid even when it is 20 Amos tall.
5. For example, the Gemara says that Rav Huna's statement was said only according to Rebbi Zeira, who says that one must sit in the shade of the Sechach. If the Sukah is wider than four by four Amos, then he will be sitting in the shade of the Sechach even though the Sukah is 20 Amos tall.