Commas serve multiple functions. You should use a comma to separate items in a series. You should also use a comma to separate the date and month from the year (April 12, 2011) and cities from states (Detroit, Michigan). Commas also follow introductory phrases and clauses (After the storm,) and can join independent clauses if a conjunction follows (She was hungry, and she wanted to eat immediately.)
Periods, question marks and exclamation points all serve as end marks, indicating the end of a sentence. If a sentence is a statement (declarative), punctuate with a period. If it is a question (interrogative), follow with a question mark. If the sentence is not a question and requires extra emphasis (imperative), you can end with an exclamation point. Avoid the use of exclamation points in formal academic and business writing, unless congratulating someone.
Semicolons join two independent clauses. When two sentences are closely related, use a semicolon to join them for better flow in your writing. You should also use semicolons to separate items that already have commas in them. For example: He visited Venice, Italy; Toledo, Spain; and Stockholm, Sweden. Use a colon to precede a list or example.
Apostrophes indicate possession or signify omitted letters within words. To illustrate possession with most singular words, place the apostrophe between the word and the letter S (the boy's class, a mechanic's shop). For most plural words, place the apostrophe after the S (the girls' department, all the players' jerseys). Apostrophes also indicate missing letters within contractions: don't (do not), won't (will not), they've (they have).
Quotation marks differentiate spoken dialogue from regular text. For example: She said, "Remember to study Chapter Three." You should also use quotation marks around the titles of articles, poems and other short works. For example: "Fire and Ice" is one of Robert Frost's more famous poems. Quotation marks sometimes indicate verbal irony. For example: Her "gourmet meal" was inedible.
Use hyphens to join two words that function together as one (part-time, all-inclusive). Dashes are longer than hyphens and suggest an interruption within the writing. For example: The house --- although old ---was solid.
You should place parentheses around additional information not structurally central to a sentence. For example: The movie (a comedy) received rave reviews. Ellipses denote missing text, often in long quotes. Insert ellipses by typing three periods alternating with spaces. For example: Smith found "the study . . . to be inconclusive." When the missing part of the text spans more than one sentence, use four periods.