Ramadhaan & Fasting

Fasting on only the 10th of Muharram due to some valid excuse

Q: I have read that it is makrooh tanzihi for one to fast on only the 10th of Muharram (without also fasting on the 9th or 11th). My questions are:

1. What is the ruling if one could only fast on the 10th due to some valid reason?

2. Should he rather not fast on the 10th?

3. What constitutes a valid reason?

4. What's the meaning and ruling of makrooh tanzihi?

Eating after sehri ends

Q: I woke up this morning and drank water after fajr. Not realizing I needed to fast today. However, since then I haven't eaten or drank and want to keep my fast. Would it be accepted? I didn't know I had to fast today and a friend told me. 

Will one receive the reward of fasting in Zul Hijjah if he keeps qadha fast in the first 10 days of Zul Hijjah?

Q:

1. If a person has fasts pending for a vow that he has made without him having specified the date, can he fast these fasts of zul hijjah or should he keep those fasts first?

2. If he has to fast for his vow in these 10 days will he acquire the sawaab of the zul hijjah fasts? Also if a woman fasts ramadaan qadhas in these days does she get the sawaab of zul hijjah fasts?

Qadha for missed fasts

Q: If I have not kept many fasts without any valid reason in my childhood, when I became baligh how do I fulfill it? Do I have to make kaffara? What is the kaffara and what if the amount of fasts is so many and I don't remember?