Halakhah for Shevuot 57:8

To not swear in vain: That we not swear pointlessly, as it is stated (Exodus 20:7), "You shall not take the name of the Lord, your God, in vain." And the notion of pointlessness has four angles: [The first is,] for example (Shevuot 29a), that he swears about something known [to be true] that it is not so, like swearing about a pillar of marble that it is a pillar of gold. And so [too], anything like this. The second angle is, for example [Talmud Yerushalmi Shevuot 3:8], that he swears about what is known to be so, like about a stone that it is a stone and about a tree that it is a tree, and all that is like this. The third angle is that he swears to negate this commandment or the commandments that God, blessed be He, commanded us; as this is also completely pointless, since it is not in his hand to swear [to negate] that which God has already obligated him - and it is like the one who swears about something known that it is not so. The fourth angle is that he swears to do something that he does not have the power to do; for example (Shevuot 25a) that he will not sleep for three consecutive days, or that he will not eat for seven consecutive days. And so [too], anything like this.

That He prohibited us - that we not make a vain oath. And that is His, may He be blessed, saying, "You shall not take the name of the Lord in vain" (Exodus 20:7). And that is that one swears about something - the existence of which is axiomatic - that it is different than what it actually is, or about the existence of something impossible or [if] he swears to negate the matter of a commandment. And likewise if he swears about [the truth of] something known, that no one denies or disagrees about at all - for example, he swears by God that anything that is slaughtered, dies - this one has also taken the Lord's name in vain. (Yerushalmi Shevuot 3) And the language of the Mishnah (Shevuot 29a) is, "Which is a vain oath? One who swears to change that which is known." And one who transgresses this negative commandment is lashed if he was volitional, and exempted if he was inadvertent - like the other ones guilty of [violation of] a negative commandment, as we explained. And there, they said in Shevuot (Shevuot 29a), "This is the vain oath on an utterance for which one is liable for lashes, when volitional; and exempted when inadvertent." And the regulations of this commandment are explained there. (See Parashat Yitro; Mishneh Torah, Oaths 1.)