The period, question mark and exclamation point alert the reader to the end of a complete sentence. Each one changes the meaning of a sentence: a period concludes a statement, a question mark lets the reader know a question is being asked and an exclamation point expresses a thought with emotion. Without proper punctuation, you have no way of knowing which words belong together to form a complete thought.
Quotation marks identify an individual’s exact words. Quotation marks appear immediately before and at the conclusion of the exact words or thoughts of another person. This punctuation leaves no room for error as to who said what and what was said. One incorrect word can change the entire meaning of a sentence.
The apostrophe stands in for missing letters in contractions, or words that are shortened in informal writing. They take the place of the missing word “of” when indicating possession. Without the apostrophe, "won’t" becomes "wont" -- an entirely different word. To indicate that something or someone belongs to something or someone, writing “Sarah’s cat” uses fewer words and is easier to understand than “the cat of Sarah” or “the cat belonging to Sarah.”
Commas used between items or phrases in a list are called serial commas. They help the reader understand the relationship between the items, such as in the list "corn, beans and tomatoes." The omission or inclusion of the comma following the word "beans" is a matter of debate among writers. Some prefer to use the comma, changing the punctuation to "corn, beans, and tomatoes." Semicolons separate items in a list of items or phrases already containing commas, such as "large, red wagon; gallon-size, reclosable plastic bag; and powdered, green tempera paint.
Punctuation separates introductory information, as in, "If you want to remain pain-free, you must exercise every day." The comma also separates non-essential information from the rest of the sentence, as in, "Jenna invited her uncle, who lives in Chicago, to her graduation party." It can link two related ideas, such as the use of a semicolon in, "Some writers use a serial comma; others omit it." Colons direct the reader’s attention to what follows or identify additional information, as in, "She was left with only one course of action: ignore her fear of heights and rescue the child."
Certain words are known more by their shortened, abbreviated form than by the full word, such as Dr., Mr., Mrs. or Rev. Using a period identifies the abbreviated form. Punctuation also facilitates the writing of dates, such as 10/22/93 or 10.22.93. It also clarifies numbers such as $1,635 or $16.35; here, what a difference the comma and period make.