To be an effective writer, you must follow certain principles of standard written English, the kind of writing used in most public communication. If sentences do not follow or conform to these guidelines, they will probably confuse or mislead your reader. In either case, you will have failed in your effort to communicate.

Standard written English requires correct sentence structure and punctuation. To understand sentence structure and to recognize and fix problems correctly, you need to know the definitions of a phrase, an independent clause, a dependent clause, and a sentence.
A phrase is a group of words that does not have a subject or a verb, or both. It does not make sense by itself.
An independent clause has a subject and a predicate, and can stand alone as a complete sentence.
A dependent clause has a subject and a predicate, but depends on an independent clause to be complete. Dependent clauses are introduced by subordinating conjunctions (after, although, because, before, if, though, unless, until, when, where, who, which, that).
A sentence is a group of words containing at least one independent clause and expressing a complete idea. It has a subject and a verb and can stand alone.
Although ordinary conversation, personal letters, and even some types of professional writing (such as newspaper stories) consist almost entirely of simple sentences, your university or college instructors will expect you to be able to use all types of sentences in your formal academic writing. Writers who use only simple sentences are like a truck drivers who do not know how to shift out of first gear: they would be able to drive a load from Montréal to Calgary (eventually), but they would have a great deal of trouble getting there.

If you use phrases and clauses carefully, your sentences will become much more interesting and your ideas, much clearer. This complex sentence develops a major, central idea and provides structured background information:

Since it involves the death not only of the title character but of the entire royal court, Hamlet is the most extreme of the tragedies written by the Elizabethan playwright William Shakespeare.
Just as a good driver uses different gears, a good writer uses different types of sentences in different situations:
a long complex sentence will show what information depends on what other information;
a compound sentence will emphasize balance and parallelism;
a short simple sentence will grab a reader's attention;
a loose sentence will tell the reader in advance how to interpret your information;
a periodic sentence will leave the reader in suspense until the very end;
a declarative sentence will avoid any special emotional impact;
an exclamatory sentence, used sparingly, will jolt the reader;
an interrogative sentence will force the reader to think about what you are writing; and
an imperative sentence will make it clear that you want the reader to act right away.
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