Making errors in your writing confuses the reader and distorts the message you are attempting to communicate. Many young writers make the mistakes listed below. Use this list to improve your writing and your ability to communicate with your reader.[1]

Try to AVOID the following...

• DO NOT use sentence fragments.

Example: The magazines that he reads on the train.

Explanation: The thought is not complete, and the reader is left to wonder what was important about the magazines.

• DO NOT allow dependent clauses to stand alone.

Example: Although Sally liked to play soccer.

Explanation: The word "although" suggests that Sally might like to do something else better or that she was unable to play even though she liked it. An independent clause is required to complete the thought and to inform the reader of the writer's purpose.

• DO NOT use run-on sentences.

Example: Although Sally liked to play soccer she also liked to play softball and basketball but she really did not have time to play sports at all because she spent all of her time practicing the piano and going to school.

Explanation: Run-on sentences have no punctuation or have improper punctuation. They are easy to spot because they run on and on and on.

• DO NOT misplace or dangle your modifiers.

Example: The magazines are on the train that he reads.

Explanation: This sounds as if someone is reading the train rather than the magazines. The sentence should read: The magazines that he reads are on the train.

• DO NOT confuse "who" and "that."

Examples: Correct—People who read are better writers. 

Incorrect—People that read are better writers.

Explanation: "Who" refers to people. "That" refers to things or concepts.

• DO NOT use commas before/after prepositional phrases (unless the prepositional phrase acts as an introductory phrase at the beginning of a sentence).

Examples: Correct—I am going into the house for a while.

Incorrect—I am going, into the house, for a while.

Correct—For example, my house is two blocks south of your house.

Explanation: Prepositions include in, on, of, about, etc. "To" is also a preposition unless it is followed by a verb which makes it an infinitive.

• DO NOT confuse "of" and "have."

Examples: I should have (verb) gone to bed earlier. The mayor of (preposition) the town walked in the parade. 

Explanation: "Of" is a preposition. "Have" is a verb.

• DO NOT use rhetorical questions. They are overdone and cliché.

Example: Have you ever wondered why the sky is blue?

Explanation: Find a better way to capture your reader's attention!

• DO NOT use a question as an opening statement for the same reason as above.

• DO NOT use slang expressions.

There may be exceptions to this rule if you are writing creatively; however, do not forget that your purpose as a writer is to communicate, not to confuse.

There are also a few things that writers SHOULD Do...

• DO italicize the titles of books and movies.

Examples: Catcher in the Rye, Jane Eyre, Star Wars

• DO place the titles of short stories, poems, or songs in "quotation marks."

Examples: "To Build a Fire," "The Star Spangled Banner"

DO place commas and periods inside of quotation marks.  (See above example.)

• DO place question marks, colons, and semicolons outside of quotation marks unless they

are part of the quoted material.

Examples: "Are you going to the store?" she asked. Do you understand what I mean by "technical"?

• DO use pronouns correctly.

It must be clear which noun the pronoun is replacing.

Example: Mary is my best friend. She is always there for me.

Explanation: It is clear that "she" refers to "Mary."

The pronoun must agree in number with the noun it is replacing.

Examples: A good writer chooses her words carefully. Good writers choose their words carefully. 

Explanation: If your noun is singular (person), then your pronoun must be singular (either "he" or "she"). If your noun is plural (people), then your pronoun must be plural (they).

[1] You may have noticed that many experienced writers do not always follow the "rules." In most cases, experienced writers break rules in order to create some sort of effect. For example, they may want to improve the flow of their writing or they may want to emphasize a certain thought. As an inexperienced writer, it is better to stick to the rules and to learn them well. Breaking a rule because you do not know better or just because you feel like it is sloppy. When you know the rules inside out, you can break them for a good reason to make your message clearer and stronger.

source: www.thewritingtutor.biz