Three Groups In WritingSkills

Purpose, strategy, and  style are decide by you. But the decision must be made within limits set by rules over which you have little control. The rules fall into three groups : grammar, usage, and mechanic.

Grammar
Grammar means the rules which structure our language. The sentence "She dresses beautifully" is gramatical. These variations are not :

Her dresses beautifully
Dresses beautifully she

The first breaks the rule that a pronoun must be in the subjective case when it is the subject of a verb. The second violates the conventional order of the English sentence : subject-verb-object. (That order is not invariable and may be altered, subject to other rules, but none of these permits the pattern :
"Dresses beautifully she.")

Grammatical rules are not the the pronouncements of teachers, editors, or other authorities. They are simply the way people speak and write, and if enough people begin to speak and write differently, the rules change.

Usage
Usage designates rules of a less basic and binding sort, concerning how we should use the language in certain situations. These sentences, for instance, violate formal usage :

She dresses beautiful.
She ain't got no dress.

Sentences like these are often heard in speech, but both break rules governing how educated people write. Formal usage dictates that when beautiful functions as an adverb it takes an-ly ending, that ain't and double negative like a in't got no or haven't got no should be avoided.

Grammar and usage are often confused. Many people would argue that the sentences above are "ungrammatical." Our distinction, however, is more useful. Grammatical rules are implicit in the speech of all who use the language. Usage rules, on the other hand, stem from and change with social pressure. Ain't, for example, was once acceptable. The adverbial use of an adjective like beautiful was common in seventeenth-century prose. Chaucer and Shakespeare use double negatives for emphasis.

The fact that usage rules are less basic than grammatical ones, however, and even that they may seem arbitrary, does not lessen their force. Most of them contribute to clarity and economy of expression. Moreover, usage applies to all levels of purpose and strategy, to informal, colloquial styles as well as to formal ones. For example, grammatically incomplete sentence (or fragments), frowned upon in formal usage,  are occasionally permissible and even valuable in informal composition. (Witness the two fragments in the student paragraph on marriage on page 8). So is regarded in formal English as subordinating conjunction which ought not to introduce a sentence. But in a colloquial style, it may work better than a more literary connective like consequently or therefore.

Mechanics
In composition mechanics refers to the appearance of words, to how they are spelled or arranged on paper. The fact that the first word of paragraph is usually indented, for example, is a matter of mechanics. These sentences violate other rules of mechanics :

she dresses beautifully
She dresses beautifully

Conventions of writing require that a sentence begin with a capital letter and end with full-stop punctuation (period, question mark, or exclamation point. Conventions of spelling require that beautifully have two ls.
The rules gathered under the heading of mechanics attempt to make writing consistent and clear. They may seem arbitrary, but they have evolved from centuries of experience. Generally they represent, if not the only way of solving a problem, an economic and efficient way.

Along with mechanics we include punctuation, a very complicated subject and by no means purely mechanical. While some punctuation is cut-and-dried, much of it falls into the province of usage or style. Later, in the chapter on punctuation, we'll discuss the distinctions between mechanical and stylistic uses of commas, dashes, and so on.