Suppose a group of people were living on an island, all using the same language, until one day the island broke in two, separated by impassable water. In one hundred years, with no contact, would the people on both halves still use the same language forms? No. Human language is always changing. Language on each half of the island would evolve with different forms and rules. Neither would be better in any absolute sense-just different. Similarly, in the United States, language variations have developed among people separated by culture, socioeconomic status, or geography.
   However, the language of the ruling class commonly comes to be regarded as standard. In the United States, the "standard" is the language of the white middle and upper classes. Forms of English developed by people of color and by people who have been poor or geographically isolated (as in Appalachia) are sometimes said to be "bad" or "incorrect" English, but such forms are only different, not bad. Each form of English has its own rules. People who say "she working" are not speaking "bad" English; they are using a different set of rules for forming the present tense.
   One of the tasks of a good education is to make students aware of these facts about language. Another task of education, however, is to prepare students to function effectively in the world where readers generally expect writers to use Edited Standard Written English (ESWE). Thus, in this class, you must use ESWE. Here is the standard I will apply:

   On finished, final, formal papers (not on drafts, in-class writings, or writing that I specifically label as informal), you must have no more than an average of two departures from ESWE per page, in any combination of the following areas:

• End-of-sentence punctuation (avoid run-on sentences, comma splices, fragments, or misuse of semicolon). Occasionally you may use a fragment or comma splice for a special effect. Label it in the margin.
• Verb forms (use ESWE rules for adding -ed and -s, for using helping verbs, and so on).
• Verb tense (avoid confusing shifts in verb tenses).
• Agreement of subject and verb.
• Pronoun form (use ESWE rules to choose between I and me, she and her, who and whom, and so on).
• Agreement of pronoun with antecedent (the antecedent is the word the pronoun refers to).
• Use of apostrophe s and the suffix -es.
• Use of quotation marks for all quoted words.
• Spelling (a typo counts as a misspelling).
• Proper sentence sense (no words omitted, scrambled, or incomprehensible).
  

From Barbara Walvoord, Effective Grading: A Tool for Learning and Assessment. Eds. Barbara E. Walvoord and Virginia Johnson Anderson. San Francisco: Jossey-Bass, 1998.