Things nullifying fasting:

1- Having sexual intercourse:

Intercourse is when the tip of the penis disappears fully into the vagina. If a fasting person has sexual intercourse with his wife, his fasting will be nullified, and he will be obliged to make up for that day, on which he had such a sexual intercourse. Moreover, one is not only obliged to make up for such a day, but also to expiate for it. The expiation for such an act is emancipating a slave, and if one does not find a slave to emancipate, one is to fast for two months consecutively, and the one unable to fast them due to a legal excuse such as being very old, permanent illness or harm will afflict his life, then one should then feed sixty poor persons with half a Saa‘ of country’s staple food for each. 

As for the sleeping person, if he has had a wet dream, there will be no blame on him and his fasting remains valid. 

2-Deliberate eating or drinking.

As for a fasting person who forgetfully eats or drinks, the validity of his fasting is by no means affected.

Fasting cannot be affected by any of the things that break fasting except if:

1- One should not have forgotten that he is fasting.

2-One should be doing that by choice. 

3- One should know the ruling

However, if one forgets, is coerced, or is ignorant of the legal ruling and does something that can break his fasting, then his fasting is still valid and he has to do nothing.

3- Menstruation and postnatal bleeding.

One may break one’s fasting during the month of Ramadan due to a legal excuse such as menstruation and travel, and may do so for an illegal such as having sexual intercourse during fasting of the like. In both cases, it is obligatory for one to make up for the missed day(s) of fasting.

It is desirable for one to hasten to make up for the missed fasting days of Ramadan so as not to be accountable for it. It is also desirable to make up for the missed days of fasting consecutively. Moreover, it is permissible for one to delay making up for fasting, as its compensation time is flexible. However, if it is the month of Sha‘ban of the following year, and there are unperformed fasting days of the previous Ramadan – provided the remaining days of Sha‘ba  are only enough for making up for those unperformed fasting days of Ramadan – one in this case has to make up for them successively. One is to make up for fasting consecutively, due to the limited time one has. However, it is impermissible to delay making up for fasting until after the following Ramadan without a legal excuse. If one delays making up for the missed fasting days of Ramadan until the following Ramadhaan begins one is to observe fasting of the present Ramadan and to make up for the missed days afterwards. As for a person who has delayed making up for such unperformed fasting due to a legal excuse that prevent him from doing so, he does not have to do anything except making up for it (i.e. there is no expiation for it). Yet, if a person has delayed making up for fasting for no legal excuse, he is to make for the missed days in addition to the feeding of a poor person for each day he had missed, with a quantity of half a Saa‘ of food in expiation for each day.

He who has a legal excuse, such as being ill, or on a journey, so that he could not make up for the missed fasting days of Ramadan, and dies after the beginning of the following Ramadan, there will be nothing on him. but, if the person delays making up for the missed fasting days for no legal excuse then dies, there must be expiation for those days to be fulfilled from his inheritance, namely feeding (on his behalf) a poor person for each fasting days he had missed.

Those who can neither perform fasting nor make up for it, such as those advanced in years and those who are hopelessly sick, Allaah The Almighty relieves them from fasting, enjoining them to feed a poor person for each unperformed fasting days instead; they are to provide half a Saa‘ of food for each day. If he dies, then feeding a poor person for each day has to be fulfilled from his inheritance.

Among those who break their fasting due to a legal excuse are travelers, sick people whose recovery is expected, and menstruating women or women in a state of postnatal bleeding. It is obligatory for people in all the aforesaid cases to make up for the unperformed fasting days. That is to say, they are to observe fasting on other days equal to the number of days they have broken their fasting on.

If the one on a journey observes fasting with great difficulty, he will be forbidden to fast, yet valid. However, if one on a journey finds only slight difficulty, it is recommended to him to break his fast and it is detestable for him not to observe fasting. In addition, if he finds not difficulty in travel, then it is better for him to fast.

A Muslim should make his intention to observe obligatory fasting from the previous night i.e. from the sunset to the sunrise. It is sufficient to have only an intention at the beginning of fasting. However, interrupting it by illness, travel, or the like, one should renew his intention.

On the other hand, it is permissible for a Muslim to intend performing general supererogatory fasts in daytime. However, specific ones such as fasting the Day of ‘Arafah, six days of Shawwaal, Day of ‘Aashuraa’ and the like, one must make intention of performing it from the night except it is a general supererogatory fasts.