Being correct goes beyond the basic grammar that language needs in order to function. Being correct means knowing the rules that a given culture has established to judge the language of individuals. Think about it: many years ago, ain’t was not considered incorrect. How might you react to someone using that word today? We speak of this as usage. There are far more usage rules than grammar rules, and they are far more difficult to master. Many of them you just have to learn; and, after you learn them, you have to use them over and over and over in order to incorporate them into your language.