THINGS THAT INVALIDATE THE FAST
Fasting(Sawm Arabic: صوم) is another unique moral and spiritual characteristic of Islam. Literally defined, fasting means to abstain "completely" from foods, drinks, intimate intercourse before the break of the dawn till sunset, during the entire month of Ramadan. Fasting is compulsorily prescribed to Muslims in the ninth month of the Muslim calendar. 
Fasting in Ramadan is a great spiritual experience, an occasion for reflection and spiritual discipline and another chance to express gratitude for Allah’s countless blessings and guidance. It is an occasion to show one’s commitment and submission to Allah Almighty.
For eleven months, Muslims eat and drink in moderation as required by Islam, but in this month of Ramadan, they renew self-control and habit modulation in resisting their desires instead of becoming slaves to them. A fasting person must overcome hunger and thirst and must keep all of his desires under control. 
Reasons which Invalidate the Fast
1. Intentional eating and drinking
2. Intentional vomiting
3. Intentional ejaculation of semen
4. Bloodletting/ Donating blood in large quantity
7. Post-childbirth bleeding
8. Sexual intercourse
If Fast is broken due to any of the above mentioned reasons (1,2,3,4 and 5), then he/she has to make up for the broken fast along with true repentance.
If Fast is broken because of Menstruation and post-childbirth bleeding, (6,7) then she has to make up for the Fasts left.
Kaffarah (expiation) for breaking fast
If Fast is broken due to sexual intercourse, he/ she has to make up the fast and pay kaffarah (expiation) which is one of the three possiblities:
All The above mentioned expiations should be backed with true repentance, without which everything is invalid.
Who is Exempt from Sawm?
1. the insane;
2. children who are not adolescent yet;
3. the elderly and chronically ill for whom fasting is unreasonably strenuous; Such persons are required to feed at least one poor person every day in Ramadan for which he or she has missed fasting.
4. pregnant women and nursing may postpone the fasting at a later time;
5. the ill and the travelers can also defer their fasting.
What does not Invalidate the fast
1. Eating or drinking by mistake, unmindful of the fast.
2. Unintentional vomiting.
3. Swallowing things which are not possible to avoid, such as one's own saliva, street dust, smoke, etc.
4. Brushing the teeth with Miswak.