Salaah

Q: In fard salaah, if a persons wudhu breaks infront of you, can the person behind him take his place by moving forward? What is the best option in this regard?

Q: Ishraaq is read 15 minutes after sunrise. How long after that is it permissible to still read ishraaq? 

Q: Can a person who is not regular with his farz namaaz be eligible to offer tahajjud namaaz?

Q: I can walk easily, can stand for a long time, but the problem is to bend my knees. So I can't make sajdah easily. My doctor advised me not to do salah by normal way? How do I perform my salah?

Q: It is written in Behishti Gohar that it is allowed to prevent a person from crossing in front of him while he is praying. However, if 'عمل کثیر' happens, then the prayer will become fasid.

What will be considered عمل کثیر ? What should the musalli do to prevent the person from crossing him?

Q: Regarding qadha salaah (missed prayers):

What is the full intention to make before one performs a qadha salaah (Note: the intention for ample salaah missed not just a single salaah). I am fearful that if my intention is incorrect my qadha salaah wont be accepted. Many books have the intention for only missing one prayer. 

To my knowledge the intention is "O Allah, I am reading the 1st fajr of those that I have missed" but I am unsure if it is correct.

Q:

1. Can a Hanafi perform asr salaah in the Shaafi asr time if leaving for a journey or while traveling?

2. If permissible, do I have to be traveling the safar distance?

Q: If during the azaan, one forgets to call out a certain phrase, then what should one do?

Q: If a person is performing six, eight, ten or twelve rakaats of tahajjud, should he perform them in sets of two, making salaam after every two rakaats, or can he perform them all together and make salaam at the end?

Q: Please explain the exact size and material of sutra which should be keep in front of the musalla.