2,622. Beginning Hilchos Shevuos (Laws of Oaths)
Hilchos Shevuos 1:1
There are four kinds of oath: an oath of expression, an oath in vain, an oath of deposit and an oath of testimony. An oath of expression is discussed in Leviticus 5:4, “When a person takes an oath, expressing with his lips, to do harm or good.” There are four sub-categories – two regarding the future and two regarding the past. For example, if one took an oath that a past event did or didn’t occur, or that he would or would not perform some future action.
Hilchos Shevuos 1:2
An oath of expression applies to deeds that one could perform in the past or in the future. Examples of the past include, “I ate,” "I threw a stone into the sea,” or “Person A spoke with person B,” or their opposites – “I didn’t eat,” “I didn’t throw…,” or “Person A didn’t speak….” Examples of the future include “I will/won’t eat” and “I will/won’t throw….” There are therefore two subcategories for the past and two for the future.