When a person intends to fast, they must know the kind of the fast, whether it's a recommended fast, a fast in terms of a vow, nadir fast, or a qada fast. But in the month of Ramadan, it is clear that one fasts with the intention of the obligatory fast of the month of Ramadan. It is even not necessary for one to specify in their Niyyah which day of the month of Ramadan they are fasting. Even if they specify the day and later realize they made a mistake, their fast is still valid. Even if they do not know or have forgotten that it is the month of Ramadan and therefore fast with another intention, their fast is valid and it is counted as the fast of the month of Ramadan. But if one did not know or had forgotten that it was the month of Ramadan and so they did not fast, but then they realize it is Ramadan in the afternoon or they had broken their fast before noon, they must still abstain from the act that invalidate the fast until sunset out of respect for the month of Ramadan and they must also observe the qada of this fast later. But if in the month of Ramadan, one intentionally makes the Niyyah for a fast other than that of the month of Ramadan, even though they know that fasting with another intention in the month of Ramadan is not permissible, their fast will be invalid regardless of what their intention has been. They are also obligated to observe the qada of this fast. That is because the Niyyah is one of the obligatory components of fasting and must last throughout the day. This is why if one happens to be indecisive as to whether or not they should continue fasting in specific obligatory fasts, such as the fast of the month of Ramadan, their fast is invalid. Even if one decides not to fast anymore or makes an intention of doing something that invalidates the fast, their fast is invalid. For instance, when one decides to submerge their entire head underwater in the swimming pool, their fast becomes invalid, even if they do not get to do so except if they are unmindful of the fact that this action will invalidate their fast. If this occurs in unspecific obligatory fasts, such as a qada fast, on the condition that one has not committed any of the invalidators of fasting, they could make the intention anew by midday and continue the fast. In recommended fasts, though, one may renew their intention up to sunset, but of course on the condition that the fast has not been broken.