Grammatical rules do change, however slightly as new trends sway public opinion away from conformity and towards a language that is becoming more synthetic and at times simplistic. There is a classic standard that has been passed down with a British base that still stands tall in spite of Americanisms and the influences of new expressions, idioms, and the import of other words. They get incorporated into the English lexicon.
As I have seen a variety of English language forms along the way and a variation in their explanation I would advise the student to pick up as many books as possible and select the ones which give a good general explanation of the individual tense, preposition, or word usage. There are bound to be a number of interpretations as the market has become more specialized and editors are interested in renewing their texts to keep up to date with new trends that arise.
People also want to buy books that have current topics and can relate modern issues. They do not want to do tedious exercises and have to read a lot of explanations to get a clear example on how to use a modifier for example.
