I am not sure if my questions went through so I will ask them again. In one of the replies, the rabbi wrote that the halachos of niddah apply to the non-married too.
What does that mean?
Also, I heard that it is better for one to know the source of a mitzva they perform, therefore, why do women have to cover their hair? and Why are women not permitted to wear men’s clothes?
1. The halachos of niddah apply before marriage with regard to the prohibition of sexual relations with a woman who is classified as a niddah (virtually all unmarried women). Of course, the particular halachos between husband and wife do not apply.
2. Women cover their hair because the Talmud extracts from a verse that the hair of a woman is considered an erva, a part of the body that must be covered. See Even Ha-Ezer 21; Orach Chaim 75.
3. The Torah writes explicitly that women may not wear men’s clothes (Devarim 22:5). This is considered immodest behavior.