Sometimes, usually in direct speech, it is appropriate to contract (or join) two words together (dropping a number of letters). Where this happens, an apostrophe is used to indicate that there are letters missing.
It's = it is
isn't = is not
we're = we are
However many letters are missing, only one apostrophe is used.
Apostrophes may also be used to indicate MISSING LETTERS when words are mispronounced in direct speech.
"'Arry's 'ad an 'ard time," he said.
"I'm sick of runnin' away," she muttered.
We regularly use apostrophes when shortening the DATE.
The Beatles came to prominence in '62.
When the war ended in '45 it was a time of mixed emotions.
Occasionally, to avoid confusion, it may be appropriate to use an apostrophe to indicate unusual plurals (but this is rare and should be avoided wherever possible).
Dot your i's and cross your t's.
Mind your p's and q's.
Introduction - Capital letters - Full stops - Question Marks - Exclamation marks - Abbreviations - Contractions - Commas - Speech marks (Quotation marks - Inverted comas) - Possessive apostrophes - Colons - Semi-colons - Brackets - Hyphens - Dashes - Obliques (slashes)