Conditions for the validity of Salaah

Q: If I wake up 15 minutes before fajr, can I pray tahajud or not? 

Q:

1. When I make wudhu, water splashes on my thighs and my pants becomes wet. So when I pray, while in the sitting position, the skin from my thighs become somewhat visible because I wear white pants. Is my prayer valid?

2. When I pray, the wind from the fan hits my clothes and the shape of my private part is visible. Also, when some people go into sajdah while wearing pants, the shape of their testes are seen, is the prayer valid?

Q: I have seen many women read salaah with tights or skinny jeans and they put a burqa over. The burqa ends around their knees and the rest of their legs are visible. Is the salaah valid?

Q: In our country, fajar prayer starts at 3.50am and magrib starts 6.50pm. What is the correct time to perform tahajjud prayer?

Q: A person goes to visit someone else and they read salaah in their house. Later on they discover that the qibla they faced in salaah was incorrect and was in the opposite direction. Do they have to repeat all the salaah they performed at the house (including in the past)?

Q: Can one read his maghrib and esha salaah in the time of maghrib before boarding the aeroplane considering that it is quite difficult for one to read his esha salaah on the aeroplane especially when there is no salaah facility on the plane?

Q: If I read tahajjud at 3 am, can I read any other nafil salaat. eg. Salaatut tasbeeh, haajat, etc. between tahajjud until fajr sets in.

Q: Is it permissible to pray salat wearing a t-shirt which contain a picture of human which is kept coverd?

Q: What is time of zawal and when does it start and end?

Q: If someone performs his salaah facing the wrong direction, away from the qiblah, does he have to repeat his salaah?