First, linguistics is descriptive while traditional grammar is prescriptive. A linguist is interested in what is said, not in what he thinks ought to be said. He describes language in all its aspects, but does not prescribe rules of "correctness". He does not believe that there is some absolute standard of correctness concerning language use which linguists or school teachers should view as their duty to maintain. Instead, he would prefer to be an observer and recorder of facts, but not a judge. He might recognize that one type of speech appears to be socially more acceptable than others because of the influence of fashion. But this will not make him think that the socially more acceptable variety can replace all the other varieties, or the old words are always better than the new ones, or vice versa. He will regard the changes in language and language use as the result of a natural and continuous process, not something to be feared.
Second, modem linguistics regards the spoken language as primary, not the written. Traditional grammarians, on the other hand, tended to emphasize, maybe over-emphasize, the importance of the written word, partly because of its permanence. Before the invention of sound recording, it was difficult for people to deal with utterances which existed only for seconds. Then, the traditional classical education was also partly to b
ame. People were encouraged to imitate the "best authors" for language usage. Many of the rules of traditional grammar apply only to the written language; they cannot be made meaningful in terms of the spoken language, without much qualification and addition.