Not to get overly philosophical, but it is the nature of language that there is no universal, eternal, provable right answer.
If we were debating a scientific question, then, at least in principle, we could perform an experiment and find the right answer. If person A says that the chemical formula for water is H2O and person B says that it is N2O, person A is right and person B is wrong, period.
But in language, if an American says that a certain word is spelled "color" and a Briton says that it is spelled "colour", there's no experiment we can do to prove that one is right and the other is wrong.
Maybe other languages have some single recognized authority who declares right and wrong usage, so if in doubt you can check that book or ask that institution and get the official right answer. But even at that, I'm sure their answers would change over time as, for example, new words are added to the language to describe new ideas or new inventions.
Regardless, there is no single recognized authority in English. There are a number of highly respected authorities. The Oxford English Dictionary is highly respected for definitions of words. The Chicago Manual of Style, the Modern Language Association Handbook, and Strunk and White's Elements of Style, are all widely respected.
A key element in the differences of opinion comes down to how you decide what the rules are. I think all serious language students agree that what is actually used by most speakers of the language is an extremely important element. If 99% of the people use a word with a certain definition, it is pretty meaningless to say that that is not a correct definition of the word. Again, it's not like science or math: If you took a poll and discovered that 99% of the people agree that the Earth is flat, sorry, it's still round. But if you took a poll and discovered that 99% of the people agree that the word "flat" means, I don't know, "made out of wood", then that is what it means, because that is what everyone agrees that it means.
A problem arises when new words are in the process of being invented, or when the accepted definition of a word is changing. Then there can be a period when 50% of the people think a word means X and the other 50% think it means Y. To take a recent example, if you asked people 50 years ago what the word "gay" meant, they would say "happy and carefree". But today it means "homosexual", and if you use the word in the old sense people will at best find it an amusing out-of-date usage, or at worst misunderstand your meaning.
Some language exerts say that common usage is the only criteria for correctness. Others -- and I agree with this second group -- say that logic and consistency are also valid criteria in deciding what is "correct". To take a current example -- not the best example but an easy one to describe -- it is becoming increasingly common in English to use "they" as a gender-neutral singular pronoun. Others say that this is wrong, that "they" is plural and should only be used as a plural because otherwise we lose the distinction between singulars and plurals. If a rule helps avoid ambiguity or assists clarity, these folks may say that it is a good and valid rule that SHOULD be obeyed even if a majority do not obey it.
And let me add that there is a third group that says that something is a rule because somebody at some time wrote it in a book or taught it to them in school, even though it is neither commonly accepted nor particularly logical. A good example of this is the rule, "Never end a sentence with a preposition." Few serious linguists recognize this as a valid rule because, (a) many English-speaking people, including many well-educated and literate people, break this "rule" all the time; and (b) It serves no rational purpose.
