When a Nafl Fast Becomes Waajib

1. A Nafl fast becomes waajib when one makes the intention of it and commences the fast.
2. Thus, if one made the intention of a Nafl fast at Subh Saadiq, “Today I am fasting,” and one broke the fast, a Qadha is necessary.
3. At night one had the intention of fasting the next day, however, one changed one’s mind before Subh Saadiq and did not fast, then no Qadha is waajib.

(Behshti Zewar)