Is strong English necessary to qualify for immigration to Canada?

Most of Canada Immigration pathway has a minimum English requirement, you must meet this requirement in order to consider eligible for immigration.  In the Federal IRCC Express Entry system English ability is an important component to scoring hence having stronger English ability would sure be an advantage in with applying in programs that is applied through the Express Entry system, if English is not very strong but do meet minimum requirements, consider provincial nomination programs which looks in desired skills and occupational background rather than just Strong English and Young age.

What is the difference between Permanent Residency & Citizenship in Canada?

While Permanent Residents of Canada can enjoy most of the legal benefits of a citizens in Canada, they are not given right to vote in government elections, and they are not given a Canada passport until they go through the citizenship process and become Canadian Citizen.  Another key difference is Permanent Resident of Canada is required to meet a resident requirement (minimum of 2 years within any 5 year period) if a permanent resident is unable to meet this requirement, permanent residency can be revoked.

Do you have to start paying Canadian Tax once you become a Permanent Resident of Canada?

For Canada, the concept of immigrational residency and taxation residency are two complete independent concepts with each having completely separate definition and requirements. Hence, you do not automatically become a Canadian  Tax resident when you receive your Canadian PR. To see if you falls under tax resident check with CRA regarding their tax residency rules or consultant a professional taxation accountant.