What should teacher do if students keep using their own language?

I used to ask myself this question when I was forced to study grammar in school. Several decades later, I am a professional writer with a strong passion for "proper" English, simply because it makes sense. And I'll tell you why.

Communication is very fragile and misuse of communication leads to misunderstanding. And misunderstanding leads to errors that can have huge consequences, including the saving or loss of life. There must be no misunderstanding when a pilot is landing a commercial jet or a surgeon is performing a delicate operation and much of the reason that these skilled individuals are able to perform their tasks successfully is that everyone around them knows exactly what they are being asked or told to do. And they understand because everyone has agreed to a common set of terms presented in a consistent manner. Without that kind of clear communication there would be chaos (and a lot of lost lives).

In the same way, we must have some common agreement about how we express ourselves in our other activities. It simply allows us to understand each other. The more fully we agree about the meanings of Words and how they are presented, the more accurately we can communicate. To do that, there must be rules. Those rules, in this instance, become "proper" English.

But there is some debate about what "proper" English really is. For example, you'll notice that I have started some of my sentences with "And", which most English teachers would tell you is a no-no. However, I use this structure to create a more conversational style of writing. Is it "proper" English? Strictly speaking, no it isn't. But this structure is commonly used in spoken English, which is what I try to emulate in my writing. I (and many others) opt to follow common usage in some areas of my writing. However, that common usage is surrounded by "proper" English and therefore prevents the communication from becoming meaningless gibberish.

One last point: "proper" English is changing all the time. And that means phrases that may not have been deemed "proper" in the past have become acceptable, simply because so many people are now using them. It pains me to think of it, but some day phrases such as "lol", "ttyl" and "brb" may someday be accepted as "proper" English.