You have three options for punctuating the end of a sentence: a period, an exclamation mark, or a question mark. Each one sets a different tone for the whole sentence: that of a statement, an outcry, or a question, respectively.
A period marks the end of a declarative sentence: a statement of fact.
A period can also be used to end an imperative sentence, i.e., a sentence that gives a command.
A question mark indicates a question, or occasionally, a statement of surprise or disbelief:
An exclamation point is used to show excitement or emphasis.
It can also follow interjections.
In both cases, it should be used sparingly. When overused, exclamation points lose their effect; they can seem silly and unprofessional.
When we’re reading out loud, our tone naturally adjusts to what the punctuation intimates: a period is relatively calm and sedate, an exclamation mark is loud and excited, and a question mark turns the end of the sentence up in wonder.
In fact, you can use the same words and the same punctuation within the sentence and change the whole meaning by using different end punctuation. I went shopping. (Ho, hum. It was uneventful.)
I went shopping! (I had the best time and bought wonderful things!)
I went shopping? (I must have had a blackout or something. I don’t remember going shopping.)
Punctuation marks are small and easy to miss, so we always follow them with a space. Some people use one space, and others use two. If you are using a particular style or format, check to see if it has any preference for one space or two. The two-space rule is a throwback to the typewriting days; two spaces are generally no longer necessary. Modern word processing adjusts fonts so that they can easily be read without having to insert double spaces. Some academic and scientific institutions, however, like to see two spaces after a period, question mark, or exclamation mark so that the work is easier to review. While the punctuation in the middle of the sentence is extremely important, it’s the punctuation at the end of the sentence that makes or breaks the writing: If the end punctuation is incorrect, the whole sentence can be incomprehensible. For more tips on how to end a sentence, check out Period, Question Mark, and Exclamation Mark.