one of my friends asked me do the content of those grammer books change all the time? or they are like permanent things and never got changed? i am totally confused about this questions, actually, i have nooooo idea, but then, i found out the answer in TKT( teaching knowledge test) book.
grammer rules describe the way that language works, but language changes over time, so grammer rules are not fixed. they change too. unfortunately, grammer rules and grammer books don't always change as quickly as language, so they are not always up to date. for example, some grammer books say that we should use 'whom' rather than 'who' after prepositions, but, in fact, except in some situations, who is generally used, in a different word order, e.g. 'I've just met the girl who I talked to on friday' is much common and accepted than 'I've just met the girl to whom I talked on friday'.
we need to keep up to date what parts of language are changing and how.
enjoy~