If the intention of fasting for the fast of Ramadan is made at night, the fast will be valid. The intention was not made at night and even in the morning one decided not to fast on that day. The day progressed considerably and only thereafter the person realized that it is not good to leave out the fard duty and therefore made the intention of fasting - even then his fast will be valid. But if he has already consumed some food in the morning, he cannot make the intention of fasting.

If a person has not consumed any food or drink, then for the fasts of Ramadan he can make the intention of fasting until one hour before mid-day.
In order to calculate mid-day, one should note the time of subah saadiq and the time of sunset. After counting the number of hours between these two times, divide this number in half. If the intention is made within this time, the fast will be valid. If this time has expired, or exceeded the limit, the fast will not be valid. This specification of "one hour" has been made as a precaution.

For the fasts of Ramadan, the following intention is sufficient: "I am fasting today". Alternatively, at night one could say: "Tomorrow I will fast". By making this intention, the fasts of Ramadan will be valid. When making the intention, a person did not specify as to whether it is a fast of Ramadan or a fard fast. Even then the fast will be valid.

In the month of Ramadan, a person made the following intention: "Tomorrow I will keep a nafl fast and I will not keep the fast of Ramadan. I will make qada of this Ramadan fast at some other time." Even then, the Ramadan fast will be considered and not the nafl fast.

A person had missed a few fasts of the previous Ramadan. The whole year passed without his making qada of these missed fasts as yet. When the next Ramadan commenced, he made the intention of making qada of those missed fasts. Even then, the fasts of this Ramadan will be considered and the qada fasts will not be valid. He will have to keep those qada fasts after Ramadan.

A person had taken a vow that if a certain work of his is fulfilled, he will keep one or two fasts for the pleasure of Allah. Wnen the month of Ramadan commenced, he made the intention of keeping those fasts which he had vowed to keep and not the fast of Ramadan. Even then, the fast of Ramadan will be considered and not that of the vowed fast. The vowed fast will have to be kept after Ramadan.
In short, if any other intention is made in the month of Ramadan, the fast of Ramadan will be considered. All other fasts will not be valid.

If the moon of Ramadan is sighted on the 29th of Sha'baan, commence fasting the following morning. However, if it is not sighted or cannot be sighted because of clouds, do not fast the next morning as long as there is a doubt as to whether Ramadan has commenced or not. Instead, complete the 30 days of Sha'baan and thereafter commence the fasts of Ramadan.

Because of clouds, the moon was not sighted on the 29th of Sha'baan. Do not even keep a nafl fast the following day. However, if a person is in the habit of fasting on Mondays and Thursdays or any other specific day, and coincidentally that day falls on the 29th, then in such a case it will be preferable to keep this fast. Thereafter, if he receives the news of the sighting of the moon on the 29th, the fast of Ramadan will be fulfilled with that very nafl fast. He does not have to make qada of it.

Because of clouds, the moon was not sighted on the 29th of Sha'baan. Do not eat or drink anything the following day until an hour before midday. If news of the sighting of the moon is received from somewhere, make the intention of fasting. If no news is received, one can commence eating and drinking.

The moon was not sighted on the 29th. Do not think that since tomorrow is not Ramadan, let me keep my qada fast of the previous year, or let me keep the fast that I had vowed to keep. On such a day, it is also makruh to keep the qada, kaffarah, and nazr (vowed) fasts. No fast should be kept on that day. If a person keeps the qada or nazr fast, and later he receives the news of the sighting of the moon; even then the fast of Ramadan will be considered. He will have to repeat the qada or nazr fast. If he does not receive any news of the sighting of the moon, the fast will be valid for whatever reason he had kept it (i.e. whether it be a qada or a nazr fast).