Can a new vow take effect when the same prohibition is already covered by an earlier vow?
This is one of the key distinctions between vows and oaths. Vows can stack — a second vow on the same object creates an additional prohibition. Oaths cannot take effect where another oath already applies.
Question 2
Why can a vow apply to a mitzvah action while an oath cannot?
A vow works by making an object forbidden — it doesn't directly override the person's obligation. An oath is taken on one's own soul, which already stands under the prior divine command, so the oath cannot nullify a mitzvah.
Question 3
A person vowed to fast on Shabbat before Shabbat began. What is the ruling?
A vow to fast on Shabbat is valid and the person is obligated to fast. The pleasure-on-Shabbat requirement is rabbinic, and the vow — which is Scriptural — takes precedence. The exception is if fasting poses a health risk.
Question 4
If a person says 'Let my speech be forbidden to me,' is this a valid vow?
Vows require objects of substance. Speech, walking, and sleep are actions of the body — there is nothing to attach the forbidden status to, so the vow has no legal effect.
Question 5
What makes 'Let walking be like a sacrifice to me' a valid vow, unlike 'Let my walking be forbidden'?
When walking is framed as an object ('let walking be like a sacrifice'), the vow is treating it as a thing to be prohibited. When phrased as a personal action ('my walking is forbidden'), there is no object for the vow to attach to — making it void.