This article should help to clarify when and how to use various marks of punctuation. Each section contains explanations and examples of common punctuation mistakes.
Comma splices (commas between independent or main clauses)
When the comma is used to separate independent clauses, there must be a conjunction connecting them. If the conjunction is not there, we have a comma splice. You can fix this mistake by using a period instead of the comma or by adding a coordinating conjunction such as "but," "and," "or," "yet," and so on.
Right: "The purse costs $75.00. I am going to buy it."
Right: "The purse costs $75.00, and I am going to buy it."
Wrong: "The purse costs $75.00, I am going to buy it."
Missing commas after set-off/introductory words or phrases
When you want to give an introduction or provide a background to a certain sentence, use a comma to signal a pause between the introductory element of a sentence and the main part of the sentence. Notice that an introductory element can be a sentence (like in the example below) or a single word (e.g., "however," "moreover," and so on).
Right: "Before going to the work, Michael stopped at the coffee shop."
Wrong: "Before going to the work Michael stopped at the coffee shop."
Missing nonrestrictive commas (commas setting off nonessential elements)
Use a comma to signal the presence of a nonrestrictive element, that is, a word, phrase, or clause that gives additional information about the preceding part of the sentence, but which can be deleted without changing the basic meaning of the sentence. If the element is in the middle of the sentence, use a comma before and after the element.
"Steve, who was the CEO of the company, was the first to speak."
"Puerto Rico was a Spanish colony until 1898, when it was ceded to the U.S."
Unnecessary commas between subjects and verbs (often after restrictive elements)
Do not use a comma with a restrictive element, that is, a word, phrase, or clause that restricts the meaning of the word or phrase it modifies. A restrictive element cannot be deleted without changing the sentence's basic meaning.
Right: "I'll return the necklace that I borrowed after I wear it tonight."
Wrong: "I'll return the necklace, that I borrowed, after I wear it tonight."
Right: "The people who set the house on fire were never caught."
Wrong: "The people, who set the house on fire, were never caught."
Unnecessary commas between compound elements that are not independent clauses
Commas in the wrong places can break a sentence into illogical segments or confuse readers with unnecessary and unexpected pauses. Do not put a comma after the main clause when a dependent (subordinate) clause follows it. Reminder: A dependent clause is a group of words that contains a subject and verb but does not express a complete thought.
Wrong: "Bob was late for work, because his alarm clock was broken."
Wrong: "The cat scratched at the door, while I was sleeping."
Mistaking "it's" or "its'" for "its"
The word "its," spelled without an apostrophe, is the possessive form of it, meaning of it or belonging to it. The word "it's," spelled with an apostrophe, is a contraction of "it is" or "it has." Even though an apostrophe usually indicates the possessive form, the possessive in this case is the one without the apostrophe.
"The car is lying on its side in the ditch. It's a white 1986 Buick."
Missing apostrophes in possessives
Apostrophes are often omitted in possessives referring to time, as in a good day's work and two weeks' vacation. To see if you need to make a possessive, turn the phrase around and make it an "of the…" phrase.
"three days' journey" = "journey of three days"
However, if the noun after "of" is a building, an object, or a piece of furniture, then no apostrophe is needed!
"room of the hotel" = "hotel room"
"door of the car" = "car door"
Missing hyphens in unit modifiers
Use a hyphen to join two or more words serving as a single adjective/unit before a noun:
"a one-way street"
"chocolate-covered peanuts"
"well-known author"
However, when compound modifiers come after a noun, they are not hyphenated:
"The peanuts were chocolate covered."
"The author was well known."
Semicolons for colons
Use a colon at the end of a business letter greeting.
Wrong: "Dear John;"
Use a colon after an independent clause when it is followed by a list, a quotation, appositive, or other idea directly related to the independent clause.
Wrong: "We have three concerns;"
Capitalization errors when quoting material
Capitalize the first letter of a direct quote when the quoted material is a complete sentence.
Jessica said, "I can't believe that Chicken of the Sea is really tuna."
However, do not use a capital letter when the quoted material is a fragment or only a piece of the original material's complete sentence.
Although Jessica really wanted to buy the new shirt, she stated that her credit card had "hit the limit."
Finally, if a direct quotation is interrupted mid-sentence, do not capitalize the second part of the quotation.
"I didn't buy the shirt," Jessica said, "but I sure wish I had."
Bonus tip: The serial comma–the comma before the conjunction in a series of three or more items–may be used or omitted. Both practices are correct as long as one or the other is followed consistently.