A full stop is placed at the end of each sentence to indicate the end of the sentence, which can be a statement, request or command. A full stop is not used at the end of a phrase or subordinate clause. Doing so does not create complete sentences.
Example: When I saw her yesterday, she was wearing a flowery hat.
NOT: When I saw her yesterday. She was wearing a flowery hat. ("When I saw her yesterday" is an adverb clause, which is not a complete sentence – a full stop should not be used to end it.
The period is used after most abbreviations:
Example: Mr., Mrs., Ms., Dr., Rev. Wed., Oct.
Most short versions of specific expressions end in a period.
Example: A.M./a.m., P.M./p.m., p.a., e.g.
The trend today is towards writing abbreviations without a period.
Example: IOU, FBI, US, UK.
Only one full stop is used if a sentence ends with an abbreviation.
Example: Her biggest ambition is to successfully complete her M.A.
The period is used to show the shortened form of a word.
Example: Opp., mo. (Written abbreviations of 'opposite', 'month')
A full stop is always placed inside quotation marks, whether or not it is part of the quotation.
Example: John said, "That runaway horse is not mine."
NOT: John said, "That runaway horse is not mine".