Knowledge

There are hundreds of useful books in many websites e.g. Kitab Al-Irshad by Al-Shaikh Al-Mufeed, The Right Path by Sharafuddin, Then I was Guided by Dr Tijani, Peshawar Nights by Sultan Al-Wa'izeen, and many others.

These websites www.Al-Islam.org

www.shiasearch.og

www.Rafed.net

www.aqaed.com 

have also many useful books.

Wassalam.

Ignorance can be unavoidable and can be avoidable. Unavoidable ignorance is of those who have to access to knowledge like those who live and die in far away forests or remote areas with out any access to learning or asking knowledgeable persons. Unavoidable ignorance  is not entitled to punishment because it was imposed on the ignorant and he had no choice.
Avoidable ignorance is a result of laziness or not caring to gain knowledge. Such ignorance is not pardoned. The narration states that sinful person will told on the Day of Judgement : Why did you fail to do good? He replies: I did not know. He will told: Why did you fail to learn? Then he will be punished.

'This means that gaining compulsory knowledge is also compulsory and those who were able to gain it,byte they opt to ignore or don't care for gaining the compulsory knowledge are responsible about their ignorance, unlike those ignorant who had no access at all to knowledge.

Wassalam.

The whole knowledge of The Unseen (Al-Ghaib) is with Allah (SWT) (Say: None in the heavens and the earth knows The (whole) Ghalib ( The Unseen) except Allah). (Sura 27, verse 65) and He grants whatever He wants to whomever He wants from His humble servants some part of that knowledge. Some prophets  get more than other prophets depending the degree of knowledge, dedication and humbleness. The greatest degree is for the Best Prophet Muhammad (SAWA) and his Holy Progeny Ahlul Bayt (AS). Allah (SWT) granted some prophets more degree than other prophets (We have granted some prophets more degrees than other prophets) (Sura 17, verse 55). The gift of the knowledge of unseen from Allah (SWT) will vary between prophet to another prophet.

Wassalam.

The status and knowledge of the Prophet Muhammad ( SAWA) is the greatest above all the Prophets. His Progeny are from him as Allah says in Quran ( A Progeny (of the Prophet) from him and he is from them (3:34).

The knowledge of the Prophet Muhammad is higher than the knowledge of all previous prophets. Since his Progeny inherited and got his knowledge, their knowledge is higher than all human beings except the Prophet Muhammad (SAWA).

Wassalam.

Islam allows studying and learning any useful knowledge in any science from non Muslims. The history of Muslims has lot of examples of non Muslim scientists in medicine and other worldly sciences who were active in Muslim society.

Muslims are not allowed to take religious knowledge from non Muslims.

Wassalam.

As far as your students are from general public who can use the knowledge in good way, and you do not know for sure that they will misuse it in unlawful ways, you are permitted to teach them and give them your skill. If you are sure that a specific group of people will use the skill in unlawful work, then you should refrain from helping them to do unlawful work.

'Wassalam.

The minimum obligatory knowledge is the basic faith (Tawheed, Adl, Nubowwah, Imamat, Qiyamah) and day to day matters which you need it like Wudhu, Ghusl, Tayammum, Salah rules and what to do when you doubt the number of Rak'ats, fasting, Khums, Zakat, Am Bil Ma'roof and Nahi Anil Munkar, Halal food rules, family rules, Hijab rules, etc.

Wassalam.

He needs to repent and decide not to commit such crime again and seek forgiveness from Allah (SWT) but he does not need to divorce his wife.

Committing such major sin is strongly forbidden and condemned and the knowledge or no knowledge of his wife does not change the rule.

Wassalam.