EDIT: you guys have dug up some truly horrible pisstakes :D Thank you for those.
To the serious folk - relax a little. This is Mildly Infuriating, not I'm dying if this doesn't stop. As a non-native speaker I was taught a certain way to use the language. The rules were not written down by me, nor the teachers - it was done by the native folk. Peace!
One important thing to realize is that different dialects of English have slightly different grammars.
One place where different dialects differ is around negation. Some dialects, like Appalachian English or West Texas English, exhibit ‘negative concord’, where parts of a sentence must agree in negation. For example, “Nobody ain’t doin’ nothing’ wrong”.
One of the most important thing to understanding a sentence is to figure out the dialect of its speaker. You’ll also notice that with sentences with ambiguous terminology like “he ate biscuits” - were they cookies, or something that looked like a scone? Rules are always contextual, based on the variety of the language being spoken.