One subject at the base of linguistics is grammar. It is the sentence order and the way we speak. What is more interesting, is the ideas behind what is correct and what is not. At base level, grammar has to exist, as it affects the meaning of words. For example, Lynne Truss’ book Eats, Shoots and Leaves, she discusses the need for correct punctuation in order to maintain intended meaning, going as far as to state “The rule is: don’t use commas like a stupid person. I mean it.” Her book is a humorous introduction to the base level need for grammar of some sort.
However, linguists are not concerned with super strict ideas such as ending sentences without prepositions as claimed by Linguistic Society of America. So what makes a native speaker say “that’s not quite right”? It can be the sounds a person makes or the way they order sentences. In a sense, casual language can still be considered grammatical if it is understandable. In end, the grammaticality – as seen by linguists – of a sentence comes down to the opinion of the native speakers.