All the so-called four letter words, are correct or standard English. They always were. They just aren't considered appropriate now. The S-word was still considered appropriate in George Washington's time. Like Ben Franklin once said: "Force shites upon Reason's Back." The F-word was considered appropriate in Anglo-Saxon times. Colloquial expressions are also not non-standard. They are correct English too. But teachers usually don't like you using them. Actually, our teacher in the 8th grade pointed out, "ain't" sounds terrible. And people rarely use it today. But it actually is the correct form of "am not". Just like "won't" is the correct form of "will not".
