All sentences begin with a capital letter (also called an upper-case letter).
All sentences end with what’s called a terminal punctuation mark: either a period (also called a full stop), a question mark, or an exclamation mark.
Periods mark the end of statements, indirect questions, and most imperative sentences. An indirect question is a sentence that asks a question but has the word order and sentence rhythm of a statement. In imperative sentences the imperative form of the finite verb is used to make requests or give advice, directions, or orders:
[Statement] Doggie bags aren’t designed to carry dogs.
[Indirect Question] He asked how the flea training was going.
[Indirect Question] I wonder if Jumbo has stopped sulking.
[Request] Please leave your geese outside.
[Order] Make sure the fleas get to bed early.
[Directions] Put your hand in the piranha tank, and tell me when you feel something.
Question marks come at the end of direct questions; that is, questions written down in the same form as you’d ask them in speaking:
Whose turn is it to bother the boa?
Are you letting the cat out of the bag?
You did turn into a prince, didn’t you?
You mean life isn’t a great big pizza?
The ducks finished all the champagne?
In the last two examples the questions are in statement form. In speaking these aloud, your voice rising at the end would show that a question is being asked, but the question marks are the only indication of this in writing.
Exclamations are direct expressions of intense feeling or strongly emphasized commands:
Kiss me, you fool!
Yikes, a flying piranha!
Are you serious! [This is an "exclamatory question": the voice goes up at the end, but the exclamation mark indicates the speaker's surprise or disbelief.]
Spacing between Sentences
In typing, we leave one space between sentences in the same paragraph: the combination of terminal punctuation mark, space, and the capital letter that begins the following sentence makes the division between sentences visually obvious. At the end of a paragraph, we leave the rest of the line empty and move down to the next line. Often, the first word of the new paragraph is indented—moved several spaces to the right of the left margin (the tab key on the computer keyboard is often used for this). Or rather than indent, we can skip lines between paragraphs.