Proper grammar, despite what many of us would LIKE to believe, is really a matter of a consensus of educated people. (Sometimes it ends up being the consensus of UNeducated people, but then that's another post.)
What's clear is that proper grammar isn't static, and words, spelling, and syntax that were acceptable in 1910 are not acceptable in 2010, and vice versa. Some of the longest held "rules" of grammar are now in flux, and I as a writer am having to adjust, which is not always easy for folks in their second half-century of life.
Today I ran across a summary of some of the sacred rules of grammar and how it is now perfectly acceptable to bend or break them. Because I tend to agree with this particular list, I encourage you to go read it, especially if you're teaching writing or grammar to children. Don't saddle them with the constraints of a previous century when they have to live and write in this one.
I don't mean by this that it's okay to have sloppy writing or grammar or to make up our own rules...just that we need to stay in tune with what's generally acceptable in educated circles so our kids don't come off as sounding pretentious or stilted when they write. That is something 'up with which we should not put' (to quote Winston Churchill). ;-)
5 Simple Grammar Tips