If a person intentionally and voluntarily does something that invalidates a fast, his fast becomes invalid; and in the event that he does not do it intentionally, there is no problem [and his fast remains valid]. However, if a junub goes to sleep and – as per the details mentioned in Ruling 1600 – he does not perform ghusl until the time of ṣubḥ prayers, his fast is invalid.
Furthermore, in the event that one does not know that some of the things mentioned previously invalidate a fast, and he has not been negligent in not knowing, nor does he doubt [that a particular thing may invalidate his fast], or he trusts in something that is legally authoritative (al‑ḥujjah al‑sharʿiyyah) [for example, the statement of a reliable person], and he does that thing, in such a case, his fast does not become invalid except in the case of eating, drinking, and sexual intercourse.