Usage Notes
Many dictionaries comment on the usage of words in different types of situations. Many of the old dictionaries contained only the standard accepted usages. "Ain`t" was not to be found and teachers could correct their charges by saying, "Ain`t isn`t in the dictionary." Imagine the teachers` dismay when dictionaries started including such forms! The dictionaries do comment on the usage--non-standard, colloquial, regionalism are some of the terms used to let readers make their own decisions as to whether they want to use the term.
One final comment--forms of speech change much more rapidly than written forms; and many spoken items never make it to the written language (slang, for example). For this reason, you will need to buy several dictionaries during your reading life. Find one that you like, and consider its new editions when it`s time for a new one.