Oaths can be of
two types: to do something, or not to do it. For example, “I will eat
this bread” or “I will not eat this bread.” These are the oaths explicitly mentioned
in the Torah. However, one can derive the same two types of oaths as
related to the past, where the person swears that he did something, or
didn't do it. All four types, if made inadvertently, require the
offender to bring a variable-type offering, which depends on his means.
(Here the Talmud digresses from oaths to discuss other cases of “two
that are in reality four.” It will not come back to oaths until after
eighteen pages.)
But why study oaths right after Makkot (lashes)? Because one of the last
subjects in Makkot was multiple penalties for one transgression, and
“two that are four” is formulated similarly.
Art: Isaac Luttichuys - Still Life with Bread and Wine Glass