Halakhah for Shevuot 41:12

To not swear falsely: To not swear falsely, as it is stated (Leviticus 19:12), "And you shall not swear in My name falsely." And they, may their memory be blessed, explained (Shevout 21a) that this verse warns about an oath of speech. And an oath of speech is what is stated in the Torah (Leviticus 5:4), "Or a soul that swears to express with the lips to do bad or to do good." And it is divided into four parts - two of the future and two of the past, such as swearing on something that was done or not done, and on something that in the future he will do or will not do. And an oath of speech is only practiced with things that it is possible for a person to do, whether in the past or in the future. How is of the past? "I ate," or "I did not eat"; and so [too,] "I threw," or "I did not throw a stone into the sea." And how is of the future? "I will eat," or "I will not eat"; or "I will throw," or "I will not throw." But with things that have a prevention from the Torah, an oath of speech is not practiced. As an oath only rests upon an optional matter - that if he wants, he does it and if he wants, he does not do it - as it is stated, "to do bad or to do good." But with any matter of a commandment, there is an obligation upon him to do it. Therefore an oath of speech does not rest upon him, whether in the past or in the future - in the case that he swears to perform a commandment, and he did not perform it; and so [too,] if he swears that he performed a commandment, and he did not perform it. As [just] like a liability [for punishment for a false oath] does not rest upon the matter of a commandment in the future, so too does it not rest upon it in the past. And so is the matter elucidated in its place in Shevuot 27a.