The mishna lists 4 topics where the Torah mentions 2 things that are forbidden and the rabbis extend it to 4, one of them being oaths that one takes that one will or will not do something, an oath of expression. Some of them incur the same type of sacrifice – a sliding one (what one brings depends on the financial means of the one obligated to bring the sacrifice). The mishna then digresses to the second category – one who goes to mikdash while impure (and forgot momentarily about being impure) or ate holy items while impure. The process of atonement through sacrifices for sins that one did unwittingly are discussed – what if one never realizes one’s mistake? What are the different categories and which sacrifices atone for which type? What is the difference between the sin offerings brought on Rosh Chodesh, the 3 holidays (regalim) and the one brought on YomKippur? What about all the other sacrifices brought on Yom Kippur and the one sent to Azazel? What is each one meant to atone for? Why do the Kohanim have their own offering with a separate confession?