Woman taking a second husband

Q: Can a married woman whose age is 27 have a second marriage due to sex and economic necessity. She can't take a divorce as her husband is ill and they also have a 6 year old child. She has not had sexual relations with her husband for the last 3 years as her husband is incapable. So she is afraid of getting involved in zina. Can she take a second husband.

Muqtadi saying the takbeer when moving to different postures

Q: Is saying transitional takbeer (Allahu Akbar) when going to ruku, sujood, qiyaam obligatory upon the individual praying behind an imam or can he just follow the imam.

I have performed many prayers behind the imam and I thought I wasn’t obligated to say the takbeer besides takbeer al ihram. Would my previous prayers be invalid otherwise even if I were unaware? 

Breaking a promise made to Allah

Q:

1. If I make a promise and say insha Allah after it, would I have to pay kaffarah if I break that promise? For example, if I say I promise not to eat a chocolate for three days insha Allah and end up eating the chocolate would I have to pay kaffarah for breaking that promise?

2. If I have to pay kaffarah, how much do I have to pay?

3. If I have to pay the kaffarah and I made a few other promises just like that and they're all broken, do I have to pay separately for each, or once for all of them?

Comprehensive tajweed kitaab

Q: Several years ago, my Qaari Saheb taught me proper, comprehensive tajweed principles along with their application. Eventually, I forgot the principles although if I am asked to recite something, I can Alhamdulillah recite it properly.

I wish to revise tajweed. Please suggest a book (in any language: English, Urdu, or Arabic) that explains the principles comprehensively.

Repeating salaah if one found out that wudhu was not valid

Q: I was sitting after performing salaah when I noticed that there is a black, sticky substance on my foot through which water could not pass. It was the size of a whole round fingernail, and I had not seen it before.

Then, my brother told me that he saw it since 3 or 4 days on my foot. I had also led jamaat during these days. Will I have to repeat all the salaah that I had offered on these days? Will those who followed me in jamaat also have to repeat their salaah?