There are two ways to look at language rules: descriptive and prescriptive. Descriptive rules are about how native speakers actually speak the language. Prescriptive rules are about how someone has decided the language should be spoken. Prescriptive language is what is taught in school and descriptive language is how most people actually talk. In the past, editors, the writers of dictionaries and other language authorities have only been interested in the prescriptive rules, but this is starting to change.
The prescriptive rules concerning contractions are quite clear. There are positive contractions in which a pronoun is blended with a verb: "I am" becomes "I'm," "I will" becomes "I'll," "I have" becomes "I've," etc. There are negative contractions in which verbs are negated: "can not" becomes "can't," "could not" becomes "couldn't," etc. There is a list of approved contractions and you can't just make up new ones.
One place where the disconnect between prescriptive and descriptive rules in English can be seen is in regional English. For example, each region has different rules for contractions—combining words by dropping syllables. In New York City, "Jeet yet?" is instantly understood to mean "Did you eat yet?" although this is definitely not part of any prescriptive curriculum. All along the Gulf Cost, "Nawlins" is the name of the city in Louisiana where the Mardi Gras takes place. If you don't use these contractions in these places, you are immediately labeled a foreigner.
Prescriptive English requires that pronouns be put in a "case" to mark their role in a sentence. For example, the words "I" and "me" refer to the same person, but "I" is used when this person is the subject of the sentence, and "me" is used when the person is the object of the sentence (following a preposition). These prescriptive rules are often ignored. Another prescriptive rule is the distinction between adverbs (modify verbs) and adjectives (modify nouns). According to prescriptive English, the adjective adds "-ly" to become an adverb—although there are plenty of irregular cases such as "good" (adjective) and "well" (adverb).
The use of prescriptive and descriptive rules is definitely associated with class and eduction. An upper-class or highly educated person in much more likely to use cases for pronouns—to say "with whom" or "to whom"—than a lower-class or uneducated person. Similarly, the prescriptive use of adverbs can be a social marker. Educated or upper-class people are more likely to say "He reads well" instead of "he reads good." Descriptive rules tend not to make a distinction between adjectives and adverbs—just to use the adjective for both.
Prescriptive English dictates that certain words be used in certain situations to make a distinction. For example, "few" means a smaller amount of something that is counted, whereas "less" means a smaller amount of something that is measured. One common place that this prescriptive rule is ignored is in grocery store express check out lines, the "10 items or less" line. It should be "10 items or fewer." Similar pairs of words are "lay" (transitive) and "lie" (intransitive) and "who" and "that" when used as pronouns in expressions like "The man that came to dinner," which should be "the man who came to dinner."
Prescriptive grammar often requires the "proper" word in a situation to make a distinction that many people simply ignore. Prescriptively, the distinction is important, but to most people most of the time, it is not.
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