What is the ruling on the hijab (head covering)? Is it permissible for a Muslim woman to uncover her hair if her clothes are modest? And is a woman considered a Muslim if she observes all the pillars of Islam, such as prayer, fasting, and zakat, but does not cover her head?

A woman's hair is 'awrah (private parts) and it is forbidden to expose it to non-mahram men, by agreement of the people of knowledge. The Islam of a woman who does not cover her head is valid, for displaying one's adornments (tabarruj) and exposing the 'awrah are not among the nullifiers of Islam, but rather a sin like all other sins. A Muslim woman who does not adhere to the hijab is threatened with curses, entry into Hellfire, and deprivation from Paradise.