When to Use a Period (Full Stop) (with Examples)

A period (.) (or full stop in the UK) is a punctuation mark used:

At the end of a declarative sentence.
At the end of an imperative sentence (which is not forceful enough for an exclamation mark).
In an abbreviation (including initialisms and contractions).

Examples of a Period (Full Stop) Ending a Declarative Sentence

Here are some declarative sentences:

Last words are for fools who haven't said enough. (Karl Marx, 1818-1883)
I never forget a face, but in your case I'll be glad to make an exception. (Groucho Marx, 1890-1977)
I am a Marxist of the Groucho tendency. (Anon)

Do not forget that a declarative sentence can include an indirect question. Do not be tempted to use a question mark. For example:

She asked if it were true.
She asked if it were true?

Examples of a Period (Full Stop) Ending an Imperative Sentence

Here are some imperative sentences ending in periods (full stops):

Please print four of these for the meeting.
Write again soon.

These are commands, but they are not forceful commands. A forceful command is followed by an exclamation mark.

Read more about imperative sentences.
Read more about exclamation marks.

Examples of a Periods (Full Stops) in Abbreviations

Here are some examples of periods (full stops) in abbreviations:

B.B.C.
a.m.
etc.
Prof. Munro
Dr. Jones

.
.

You Can't Put a Comma and Carry on Writing

Remember, a sentence expresses a complete thought. A sentence must contain a subject and a verb (even if one is implied).

Once you have written a sentence, you cannot put a comma and carry on writing. This is called a run-on error or a comma fault. For example:

I love angel cake, it is my favourite.