What is the true meaning of the expression "the rules of grammar"? Discuss whether these "rules" are unchangeable or subject to change with the passage of time.
I don't know that there is one answer to the true meaning of the 'rules of grammar' but in my own words, I'd describe it as a set of normative guidelines that govern the way a language can be properly put together.
That said, I think some 'rules of grammar' (for the English language) are set in stone. We are never, for example, likely to start putting our adjectives behind our nouns, as so many other languages do. However, I think dialect and culture both affect some rules of grammar that are able to change. How else could we account for words like "ya'll" and phrases like "fixin' to" as acceptable English? *I actually think I've broken a grammar rule in that last sentence.*
You also should consider that there are different 'rules of grammar' for speaking and for writing. It is acceptable to say, "The professor teaches their class with gusto," (especially if you are attempting to not give away whether the prof is male or female), however, in writing it must be put like this: "The professor teaches his (or her) class with gusto."
There are also different 'rules of grammar' for different styles of writing. There are different sets of rules for fiction, poetry, prose, drama, etc. Then there is another set of rules for professional/technical writing (which is also constantly changing by the way, just look at how many editions exist of the MLA Handbook).