it says ha'oseh sukkaso tachas ha'ilan ke'ilu assaah b'toch habais. - sukkah tes). its mashma that making a sukkah in a house is very pashut. but both dinim of a sukkah in a house and a sukkah under a tree are learnt from the same limud. - bc in the passuk it says sukkos without a vov. so why is making a sukkah in a house more pashut/understandable than making it under a tree?!
chaim,
The Tosfos Yom Tov asks your question. He explains that while they are from the same Limud, they are two different things. A Bayis, by nature, is an enclosed house with a closed roof. Accordingly, any Sukah made under anything that we know as a house (without getting into strange treehouses and the like) is going to be Pasul. This is simply incorrect regarding a tree. Some trees are known as typical trees, even though they are Chamaso Merubah. Additionally, it is possible to cut off branches of trees and change them into Sechach Kasher. For all of these reasons, the Mishnah says that a Sukah under a tree is only Pasul if the tree is like the typical Bayis that we are discussing (Tzilaso Merubah, no possibility of turning into Sechach Kasher, etc.).
All the best,
Yaakov Montrose