Things which invalidate the fast
What actions invalidate the fast?
Whoever eats or drinks deliberately during the fasting hours of Ramadan invalidates his fast according to scholarly consensus.
Making up a missed fast day
Scholars of some schools of jurisprudence maintain that a person who deliberately breaks his fast must only make up the day he broke his fast while others maintain that such a person must make it up in conjunction with expiation.
Expiation for deliberately breaking the fast
A person who deliberately breaks his fast must fast 60 days consecutively. If unable to do so, then he must feed 60 poor persons. All scholars are unanimous that a person who deliberately eats or drinks during the fasting hours of Ramadan is blameworthy for violating the sanctity of the rite.
Eating or drinking absentmindedly
According to the opinion of the Maliki school, a person who eats or drinks absentmindedly must continue his fast and only make up the missed fast day. Scholars of the other schools maintain that eating or drinking absentmindedly does not invalidate the fast and the fast of such a person is valid; he does not have to make it up. This is the preponderant opinion.
Things which invalidate fasting
1) Deliberate sexual intercourse during the fasting hours of Ramadan (by deliberate we mean that one knows a certain act is unlawful during an obligatory fast and remembers that one is fasting but does it anyway). According to all schools of jurisprudence, such a person must make up the missed fast day as well as expiation. Some schools maintain that both spouses are to make expiation. Others maintain that only the husband must do so and that the wife must only make up the fast even though both are blameworthy and share the sin of disobedience.
2) Deliberate vomiting.
3) Any liquids or solids that reach the body cavity invalidate fasting. It is a condition in the Hanafi and Maliki schools that solids must settle in the stomach to invalidate fasting.
As for applying kohl: According to some scholars, the fast is invalidated if kohl is applied during the fasting hours of Ramadan and one can feel its taste in the throat or mouth. Abu Hanifa and al-Shafi'i (may God be pleased with them) maintain that kohl does not invalidate the fast, even if it is applied during the fasting hours of Ramadan. They base their opinion on reports narrating that the Prophet (peace and blessings be upon him) used to apply kohl during the fasting hours of Ramadan. This is the preponderant opinion in our view.
And God, the Almighty, knows best.