Chapter 2, Mishnah 1. The previous mishnah closed with a mention of the laws of arayos, the forbidden relationships, and for that reason our mishnah opens with a matter connected to arayos and to the safeguards that the Sages established in their study.
"Ein dorshin ba'arayos bishloshah":
A teacher does not expound and explain the subject of arayos, the forbidden relationships, in front of three students. The Gemara explains that this refers to the "hidden" arayos, and Rashi explains that these are prohibitions not stated explicitly in the Torah but derived through exposition, among them:
His daughter born to a woman he is not married to.
His father-in-law's mother.
His mother-in-law's mother.
The reason, as the Gemara explains: there is a concern that while one student is engaged in give and take with the teacher, the two remaining students will get involved in a fine halachic discussion between themselves and will not be listening to the teacher. As a result, they are liable to come to a mistake and permit something that is forbidden. The Sages raised this concern specifically in this area and not in other parts of the Torah, because a person has a weakness with regard to these prohibitions.
In front of two students, however, it is permitted to expound: even if one of them becomes absorbed in discussion with the teacher, the second, who has no one to talk with, will lend an ear to their words and will not miss a thing.
"Velo bema'aseh bereishis bishnayim":
A teacher does not expound ma'aseh bereishis, the account of creation, in front of two students, but only in front of a single student. This law is derived from a verse.
"Velo bamerkavah beyachid":
The Merkavah is the description of the holy chariot of the Holy One, blessed is He, described at the beginning of the book of Yechezkel. In this subject one may not teach even a single student, "ela im ken hayah chacham u'mevin mida'ato" - unless he is wise and understands on his own, a student who grasps a matter from a mere hint and attains ideas through his own powers. For such a student it is enough to hand over the chapter headings, and he completes the rest by himself.
"Kol hamistakel be'arba'ah devarim":
Anyone who delves into these four areas - "ra'ui lo ke'ilu lo ba la'olam" - it would have been fitting for him never to have come into the world at all. And these are they:
What is above - above the heavens, above the universe.
What is below.
What was before the world was created.
What is destined to come afterward.
These are matters upon which a human being cannot truly focus his mind, and it is not fitting for a person to invest his time in them.
The same applies to one who is not careful with the honor of his Maker, the honor of the Holy One, blessed is He. The Gemara explains that this refers to one who commits a sin in secret, as if to say: the Holy One, blessed is He, does not see me. About him too it is said that it would have been fitting for him never to have come into the world.